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LIGHTNING,THUNDER 

AND 

LIGHTNING   CONDUCTORS. 

1 

1 

BY 

GERALD  MOLLOY,  D.D.,  D.Sc 

ILLUSTRATED. 

C?S§^^g^^^^^2^^^^^^^^S§5g^S^^g^ 

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I  THE  HUMBOLDT  PUBLlSHiNQ  COHPANY  I 

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LIGHTNING,  THUNDER 


AND 


LIGHTNING  CONDUCTORS. 

WITH  AN  APPENDIX  ON  THE  RECENT  CONTROVERSY 

ON  LIGHTNING  CONDUCTORS. 

*  \ 

BY 

GERALD IMOLLOY,  D.  D.,  D.SC 


ILLUSTRATED. 


NEW   YORK  : 
THE    HUMBOLDT   PUBLISHING    CO., 

28   LAFAYETTE   PLACE. 


LIGHTNING,    THUNDER,  AND  LIGHTNING 
CONDUCTORS. 

CONTENTS. 


LECTURE  I. 

LIGHTNING   AND   THUNDER. 

Identity  of  Lightning  and  Electricity — Franklin's  Experiment — Fatal  Experiment  of 
Richman — Immediate  Cause  of  Lightning — Illustration  from  Electric  Spark 
— What  a  Flash  of  Lightning  Is — Duration  of  a  Flash  of  Lightning — Expe- 
riments of  Professor  Rood — Wheatstone's  Experiments — Experiment  with 
Rotating  Disc — Brightness  of  a  Flash  of  Lightning — Various  Forms  of 
Lightning — Forked  Lightning,  Sheet  Lightning,  Globe  Lightning — St. 
Elmo's  Fire — Experimental  Illustration — Origin  of  Lightning — Length  of  a 
Flash  of  Lightning — Physical  Cause  of  Thunder — Rolling  of  Thunder — 
Succession  of  Peals — Variation  of  Intensity — Distance  of  a  Flash  of  Light- 
ning, ...........  Pages  5-26 

LECTURE  II. 

LIGHTNING   CONDUCTORS. 

Destructive  Effects  of  Lightning — Destruction  of  Buildings — Destruction  of 
Ships  at  Sea — Destruction  of  Powder  Magazines — Experimental  Illustra- 
tions— Destruction  of  Life  by  Lightning — The  Return  Shock — Franklin's 
Lightning  Rods — Introduction  of  Lightning  Rods  into  England — The  Bat- 
tle of  Balls  and  Points — Functions  of  a  Lightning  Conductor — Conditions 
of  a  Lightning  Conductor — Mischief  Done  by  Bad  Conductors — Evil  Effects 
of  a  Bad  Earth  Contact — Danger  from  Rival  Conductors — Insulation  of 
Lightning  Conductors — Personal  Safety  in  a  Thunder  Storm — Practical 
Rules — Security  Afforded  by  Lightning  Rods,  ,  .  .  Pages  26-53 

APPENDIX. 

RECENT    CONTROVERSY    ON    LIGHTNING   CONDUCTORS. 

Tfteory  of  Lightning  Conductors  Challenged — Lectures  of  Professor  Lodge — 
Short  Account  of  his  Views  and  Arguments — Effect  of  Self-induction  on  a 
Lightning  Rod — Experiment  on  the  Discharge  of  a  Leyden  Jar — Outer  Shell 
only  of  a  Lightning  Rod  Acts  as  a  Conductor — Discussion  at  the  Meeting  of 


CONTENTS. 

the  British  Association,  September,  1888 — Statement  by  Mr.  Preece — Lord 
Rayleigh  and  Sir  William  Thomson — Professor  Rowland  and  Professor 
Forbes — M.  de  Fonvielle,  Sir  James  Douglass,  and  Mr.  Symons — Reply  of 
Professor  Lodge — Concluding  Remarks  of  Professor  Fitzgerald,  President 
of  the  Section — Summary  Showing  the  Present  State  of  the  Ques- 
tion, , Pages  55-62 


LIST  OF  ILLUSTRATIONS. 

PAGE 

THE  ELECTRIC  SPARK  :  A  TYPE  OF  A  FLASH  OF  LIGHTNING,  ,        .        .      8 

CARDBOARD  Disc  WITH  BLACK  AND  WHITE  SECTORS  ;  AS  SEEN  WHEN  AT  REST,     12 

SAME  Disc  ;  AS  SEEN  WHEN  IN  RAPID  ROTATION, 12 

THE  BRUSH  DISCHARGE,  ILLUSTRATING  ST.  ELMO'S  FIRE,          .        .        .        .17 

ORIGIN  OF  SUCCESSIVE  PEALS  OF  THUNDER 22 

VARIATIONS  OF  INTENSITY  IN  A  PEAL  OF  THUNDER, 24 

DISCHARGE  OF  LEYDEN  JAR  BATTERY  THROUGH  THIN  WIRES,  .        .        .        .27 
GLASS  VESSEL  BROKEN  BY  DISCHARGE  OF  LEYDEN  JAR  BATTERY,      .        .        .32 

GUN  COTTON  SET  ON  FIRE  BY  ELECTRIC  SPARK, 33 

VOLTA'S  PISTOL  ;  EXPLOSION  CAUSED  BY  ELECTRIC  SPARK,        .        .        .        .34 

THE  RETURN  SHOCK  ILLUSTRATED, 35 

PROTECTION  FROM  LIGHTNING  BY  A  CLOSED  CONDUCTOR, 48 

INDUCTION  EFFECT  OF  LEYDEN  JAR  DISCHARGE, 56 


LECTURE  I. 


LIGHTNING     AND     THUNDER. 

THE  electricity  produced  by  an  ordinary  electric  machine  exhibits, 
under  certain  conditions,  phenomena  which  bear  a  striking  re- 
semblance to  the  phenomena  attendant  on  lightning.  In  both  cases 
there  is  a  flash  of  light ;  in  both  there  is  a  report,  which,  in  the 
case  of  lightning,  we  call  thunder ;  and,  in  both  cases,  intense  heat  is 
developed,  which  is  capable  of  setting  fire  to  combustible  bodies. 
Further,  the  spark  from  an  electric  machine  travels  through  space 
with  extraordinary  rapidity,  and  so  does  a  flash  of  lightning ;  the 
spark  follows  a  zig-zag  course,  and  so  does  a  flash  of  lightning;  the 
spark  moves  silently  and  harmlessly  through  metal  rods  and  stout 
wires,  while  it  forces  its  way,  with  destructive  effect,  through  bad 
conductors,  and  it  is  so,  too,  with  a  flash  of  lightning.  Lastly,  the 
electricity  of  a  machine  is  capable  of  giving  a  severe  shock  to  the 
human  body;  and  we  know  that  lightning  gives  a  shock  so  severe  as 
usually  to  cause  immediate  death.  For  these  reasons  it  was  long 
conjectured  by  scientific  men  that  lightning  is,  in  its  nature,  identical 
with  electricity;  and  that  it  differs  from  the  electricity  of  our  machines 
only  in  this,  that  it  exists  in  a  more  powerful  and  destructive  form. 

Identity  of  Lightning  and  Electricity. — But  it  was  reserved 
for  the  celebrated  Benjamin  Franklin  to  demonstrate  the  truth  of  this 
conjecture  by  direct  experiment.  He  first  conceived  the  idea  of 
drawing  electricity  from  a  thundercloud  in  the  same  way  as  it  is 
drawn  from  the  conductor  of  an  electric  machine.  For  this  purpose 
he  proposed  to  place  a  kind  of  sentry-box  on  the  summit  of  a  lofty 
tower,  and  to  erect,  on  the  sentry-box,  a  metal  rod,  projecting  twenty 
or  thirty  feet  upward  into  the  air,  pointed  at  the  end,  and  having  no 
electrical  communication  with  the  earth.  He  predicted  that  when  a 
thundercloud  would  pass  over  the  tower,  the  metal  rod  would  become 
charged  with  electricity,  and  that  an  observer,  stationed  in  the  sentry- 
box,  might  draw  from  it,  at  pleasure,  a  succession  of  electric  sparks. 

With  the  magnanimity  of  a  really  great  man,  Franklin  published 
this  project  to  the  world  ;  being  more  solicitous  to  extend  the  domain 
of  science  by  new  discoveries,  than  to  secure  for  himself  the  glory  of 


6  LIGHTNING  AND  THUNDER. 

having  made  them.  The  project  was  set  forth  in  a  letter  to  Mr. 
Collinson,  of  London,  which  bears  date  July  29,  1750,  and  which,  in 
the  course  of  a  year  or  two,  was  translated  into  the  principal  lan- 
guages of  Europe.  Two  years  later  the  experiment  suggested  by 
Franklin  was  made  by  Monsieur  Dalibard,  a  wealthy  man  of  science, 
at  his  villa  near  Marly-la- Ville,  a  few  miles  from  Paris.  In  the  mid- 
dle of  an  elevated  plain  Monsieur  Dalibard  erected  an  iron  rod,  forty 
feet  in  length,  one  inch  in  diameter,  and  ending  above  in  a  sharp 
steel  point.  The  iron  rod  rested  on  an,  insulating  support,  and  was 
kept  in  position  by  means  of  silk  cords. 

In  the  absence  of  Monsieur  Dalibard,  who  was  called  by  business 
to  Paris,  this  apparatus  was  watched  by  an  old  dragoon,  named 
Coiffier;  and  on  the  afternoon  of  the  tenth  of  May,  1752,  he  drew 
sparks  from  the  lower  end  of  the  rod  at  the  time  that  a  thundercloud 
was  passing  over  the  neighborhood.  Conscious  of  the  importance 
that  would  be  attached  to  this  phenomenon,  the  old  dragoon  sum- 
moned, in  all  haste,  the  prior  of  Marly  to  come  and  witness  it.  The 
prior  came  without  delay,  and  he  was  followed  by  some  of  the  prin- 
cipal inhabitants  of  the  village.  In  the  presence  of  the  little  group, 
thus  gathered  together,  the  experiment  was  repeated — electric  sparks 
were  again  drawn,  in  rapid  succession,  from  the  iron  rod;  the  prediction 
of  Franklin  was  fulfilled  to  the  letter;  and  the  identity  of  lightning  and 
electricity  was,  for  the  first  time,  demonstrated  to  the  world. 

Franklin's  Experiment. — Meanwhile  Franklin  had  been  wait- 
ing, with  impatience,  for  the  completion  of  the  tower  of  Christchurch, 
in  Philadelphia,  on  which  he  intended  to  make  the  experiment  him- 
self. He  even  collected  money,  it  is  said,  to  hasten  on  the  building. 
But,  notwithstanding  his  exertions,  the  progress  of  the  tower  was 
slow;  and  his  active  mind,  which  could  ill  brook  delay,  hit  upon 
another  expedient,  remarkable  alike  for  its  simplicity  and  for  its  com- 
plete success.  He  constructed  a  boy's  kite,  using,  however,  a  silk 
pockethandkerchief,  instead  of  paper,  that  it  might  not  be  damaged 
by  rain.  To  the  top  of  the  kite  he  attached  a  pointed  iron  wire  about 
a  foot  long,  and  he  provided  a  roll  of  hempen  twine,  which  he  knew 
to  be  a  conductor  of  electricity,  for  flying  it.  This  was  the  apparatus 
with  which  he  proposed  to  explore  the  nature  of  a  thundercloud. 

The  thundercloud  came  late  in  the  afternoon  of  the  fourth  of  July? 
1752,  and  Franklin  sallied  out  with  his  kite,  accompanied  by  his  son, 
and  taking  with  him  a  common  door-key  and  a  Leyden  jar.  The  kite 
was  soon  high  in  air,  and  the  philosopher  awaited  the  result  of  his 
experiment,  standing,  with  his  son,  under  the  lee  of  a  cowshed,  partly 
to  protect  himself  from  the  rain  that  was  coming,  and  partly,  it  is 
said,  to  shield  himself  from  the  ridicule  of  passers-by,  who,  having  no 
sympathy  with  his  philosophical  speculations,  might  be  inclined  to 
regard  him  as  a  lunatic.  To  guard  against  the  danger  of  receiving  a, 


LIGHTNING  AND  THUNDER.  7 

flash  of  lightning  through  his  body,  he  held  the  kite  by  means  of  a 
silk  ribbon,  which  was  tied  to  the  door-key,  the  door-key  being  itself 
attached  to  the  lower  end  of  the  hempen  string. 

A  flash  of  lightning  soon  came  from  the  cloud,  and  a  second,  and  a 
third;  but  no  sign  of  electricity  could  be  observed  in  the  kite,  or  the 
hempen  cord,  or  the  key.  Franklin  was  almost  beginning  to  despair 
of  success,  when  suddenly  he  noticed  that  the  little  fibres  of  the  cord 
began  to  bristle  up,  just  as  they  would  if  it  were  placed  near  an  elec- 
tric machine  in  action.  He  presented  the  door-key  to  the  knob  of  the 
Leyden  jar,  and  a  spark  passed  between  them.  Presently  a  shower 
began  to  fall;  the  cord,  wetted  by  the  rain,  became  a  better  con- 
ductor than  it  had  been  before,  and  sparks  came  more  freely.  With 
these  sparks  he  now  charged  the  Leyden  jar,  and  found,  to  his  intense 
delight,  that  he  could  exhibit  all  the  phenomena  of  electricity  by 
means  of  the  lightning  he  had  drawn  from  the  clouds. 

In  the  following  year  a  similar  experiment,  with  even  more  striking 
results,  was  carried  out,  in  France,  by  de  Romas.  Though  it  is  said 
he  had  no  knowledge  of  what  Franklin  had  done  in  America,  he,  too, 
used  a  kite;  and,  with  a  view  of  making  the  string  a  better  conductor, 
he  interlaced  with  it  a  thin  copper  wire.  Then,  flying  his  kite  in  the 
ordinary  way,  when  it  had  risen  to  a  height  of  about  550  feet,  he  drew 
sparks  from  it  which,  we  are  told,  were  upwards  of  nine  feet  long, 
and  emitted  a  sound  like  the  report  of  a  pistol. 

Fatal  Experiment  of  Richman. — There  can  be  no  doubt  that 
experiments  of  this  kind,  made  with  the  electricity  of  a  thunder- 
cloud, were  extremely  dangerous  ;  and  this  was  soon  proved  by  a  fatal 
accident.  Professor  Richman,  of  St.  Petersburgh,  had  erected  on  the 
roof  of  his  house  a  pointed  iron  rod,  the  lower  end  of  which  passed 
into  -a  glass  vessel,  intended,  as  we  are  informed,  to  measure  the 
strength  of  the  charge  which  he  expected  to  receive  from  the  clouds. 
On  the  sixth  of  August,  1753,  observing  the  approach  of  a  thunder- 
storm, he  hastened  to  his  apparatus  ;  and  as  he  stood  near  it,  with  his 
head  bent  down,  to  watch  the  effect,  a  flash  of  lightning  passed 
through  his  body  and  killed  him  on  the  spot.  This  catastrophe  served 
to  fix  public  attention  on  the  danger  of  such  experiments,  and  gave  oc- 
casion to  the  saying  of  Voltaire :  "  There  are  some  great  lords  whom 
we  should  always  approach  with  extreme  precaution,  and  lightning  is 
one  of  them."1  From  this  time  the  practice  of  making  experiments 
directly  with  the  lightning  of  the  clouds  seems  to  have  been,  by  com- 
mon consent,  abandoned. 

Immediate  Cause  of  Lightning. — And  now,  having  set  before 
you  some  of  the  most  memorable  experiments  by  which  the  identity 
of  lightning  and  electricity  has  been  demonstrated,  I  will  try  to  give 

1  u  II  y  a  des  grands  seigneurs  dont  il  ne  faut  approcher  qu'avec  d'extremes  precautions.  Le  ton- 
nerre  est  de  ce  nombre." — Diet.  Philos.  art.  Foudre. 


8  LIGHTNING  AND  THUNDER. 

you  a  clear  conception  regarding  the  immediate  cause  of  lightning,  so 
far  as  the  subject  is  understood  at  the  present  day  by  scientific  men. 
You  know  that  there  are  two  kinds  of  electricity,  which  are  called  posi- 
tivc  and  negative j  and  that  each  of  them  repels  electricity  of  the 
same  kind  as  itself,  while  it  attracts  electricity  of  the  opposite  kind. 
Now,  every  thundercloud  is  charged  with  electricity  of  one  kind  or  the 
other,  positive  or  negative  ;  and,  as  it  hovers  over  the  earth,  it  devel- 
ops, by  what  is  called  induction,  or  influence,  electricity  of  the  opposite 
kind  in  that  part  of  the  earth  which  is  immediately  under  it.  Thus 
we  have  two  bodies — the  cloud  and  the  earth — charged  with  opposite 
kinds  of  electricity,  and  separated  by  a  stratum  of  the  atmosphere. 
The  two  opposite  electricities  powerfully  attract  each  other  ;  but  for 
a  time  they  are  prevented  from  rushing  together  by  the  intervening 
stratum  of  air,  which  is  a  non-conductor  of  electricity,  and  acts  as  a 
barrier  between  them.  As  the  electricity,  however,  continues  to  accu- 
mulate, the  attraction  becomes  stronger  and  stronger,  until  at  length 
it  is  able  to  overcome  the  resistance  of  this  barrier  ;  a  violent  disrup- 


THE   ELECTRIC   SPARK  ;   A   TYPE   OF   A    FLASH    OF   LIGHTNING. 

tive  discharge  then  takes  place  between  the  cloud  and  the  earth,  and 
the  flash  of  lightning  is  the  consequence  of  the  discharge.     , 

The  whole  phenomenon  may  be  illustrated,  on  a  small  scale,  by 
means  of  this  electric  machine  of  Carry's  which  you  see  before  you. 
When  my  assistant  turns  the  handle  of  the  machine  negative  elec- 
tricity is  developed  in  that  large  brass  cylinder,  which  in  our  experi- 
ment will  represent  the  thundercloud.  At  a  distance  of  five  or  six 
inches  from  the  cylinder  I  hold  a  brass  ball,  which  is  in  electrical  com- 
munication with  the  earth  through  my  body.  The  electrified  brass 
cylinder  acts  by  induction,  or  influence  on  the  brass  ball,  and  develops 
in  it,  as  well  as  in  my  body,  a  charge  of  positive  electricity.  Now,  the 
positive  electricity  of  the  ball  and  the  negative  electricity  of  the 
cylinder  are  mutually  attracting  each  other,  but  the  intervening 
stratum  of  air  offers  a  resistance  which  prevents  a  discharge  from 
taking  place.  My  assistant,  however,  continues  to  work  the  machine  ; 
the  two  opposite  electricities  rapidly  accumulate  on  the  cylinder  and 
the  ball ;  at  length  their  mutual  attraction  is  strong  enough  to  over- 


LIGHTNING  AND  THUNDER.  9 

come  the  resistance  interposed  between  them  ;  a  disruptive  discharge 
follows,  and  at  the  same  moment  a  spark  is  seen  to  pass,  accompanied 
by  a  sharp  snapping  report. 

This  spark  is  a  miniature  flash  of  lightning  ;  and  the  snapping  re- 
port is  a  diminutive  peal  of  thunder.  Furthermore,  at  the  moment 
the  spark  passes  you  may  observe  a  slight  convulsive  movement  in  my 
hand  and  wrist.  This  convulsive  movement  represents,  on  a  small 
scale,  the  violent  shock,  generally  fatal  to  life,  which  is  produced  by 
a  flash  of  lightning  when  it  passes  through  the  body. 

I  can  continue  to  take  sparks  from  the  conductor  as  long  as  the 
machine  is  worked  ;  and  it  is  interesting  to  observe  that  these  sparks 
follow  an  irregular  zig-zag  course,  just  as  lightning  does.  The  reason 
is  the  same  in  both  cases  :  a  discharge  between  two  electrified  bodies 
takes  place  along  the  line  of  least  resistance  ;  and,  owing  to  the  vary- 
ing condition  of  the  atmosphere,  as  well  as  of  the  minute  particles  of 
matter  floating  in  it,  the  line  of  least  resistance  is  almost  always  a 
zig-zag  line.  >^\ 

jL-What  a  Flash  of  Lightning  is. — Lightning,  then,  may  be  VJ/ 
j^aqpceived  as  an  electrical  discharge,  sudden  and  violent  in  its  charac- 
ter, which  takes  place,  through  the  atmosphere,  between  two  bodies 
highly  charged  with  opposite  kinds  of  electricity.  Sometimes  this 
electrical  discharge  passes,  as  I  have  said,  between  a  cloud  and  the 
earth  ;  sometimes  it  passes  between  one  cloud  and  another  ;  some- 
times, on  a  smaller  scale,  it  takes  place  between  the  great  mass  of  a 
cloud  and  its  outlying  fragments. 

But,  if  you  ask  me  in  what  the  discharge  itself  consists,  I  am 
utterly  unable  to  tell  you.  It  is  usual  to  speak  and  write  on  this  sub- 
ject as  if  electricity  were  a  material  substance,  a  very  subtle  fluid,  and 
as  if,  at  the  moment  the  discharge  takes  place,  this  fluid  passes  like  a 
rapid  stream,  from  the  body  that  is  positively  electrified  to  the  body 
that  is  negatively  electrified.  But  we  must  always  remember  that  this 
is  only  a  conventional  mode  of  expression,  intended  chiefly  to  assist 
our  conceptions,  and  to  help  us  to  talk  about  the  phenomena.  It  does 
not  even  profess  to  represent  the  objective  truth.  All  that  we  know 
for  certain  is  this  :  that  immediately  before  the  discharge  the  two 
bodies  are  highly  electrified  with  opposite  kinds  of  electricity  ;  and, 
that  immediately  after  the  discharge,  they  are  found  to  have  returned 
to  their  ordinary  condition,  or,  at  least,  to  have  become  less  highly 
electrified  than  they  were  before. 

The  flash  of  light  that  accompanies  an  electric  discharge  is  often  * 
supposed  to  be  the  electricity  itself,  passing  from  one  body  to  the 
other.  But  it  is  not ;  it  is  simply  an  effect  produced  by  the  discharge. 
Heat  is  generated  by  the  expenditure  of  electrical  energy,  in  over- 
coming the  resistance  offered  by  the  atmosphere  ;  and  this  heat  is  so 
intense,  that  it  produces  a  brilliant  incandescence  along  the  path  of 


io  LIGHTNING  AND  THUNDER. 

the  discharge.  When  a  spark  appears,  for  example,  between  the  con- 
ductor of  the  machine  and  this  brass  ball,  it  can  be  shown,  by  very 
satisfactory  evidence,  that  minute  particles  of  these  solid  bodies  are 
first  converted  into  vapor,  and  then  made  to  glow  with  intense  heat. 
The  gases,  too,  of  which  the  air  is  composed,  and  the  solid  particles 
floating  in  the  air,  are  likewise  raised  to  incandescence.  So,  too,  with 
lightning  ;  the  flash  of  light  is  due  to  the  intense  heat  generated  by 
the  electrical  discharge,  and  owes  its  character  to  the  composition 
and  the  density  of  the  atmosphere  through  which  the  discharge  passes. 

Duration  of  a  Flash  of  Lightning. — How  long  does  a  flash 
of  lightning  last  ?  You  are  aware,  I  dare  say,  that  when  an  impres- 
sion of  light  is  made  on  the  eye,  the  impression  remains  for  a  sensible 
interval  of  time,  not  less  than  the  tenth  of  a  second,  after  the  source 
of  light  has  been  extinguished  or  removed.  Hence  we  continue,  in 
fact,  to  see  the  light,  for  at  least  the  tenth  of  a  second,  after  the  light 
has  ceased.  Now,  if  you  reflect  how  brief  is  the  moment  for  which  a 
flash  of  lightning  is  visible,  and  if  you  deduct  the  tenth  of  a  second 
from  that  brief  moment,  you  will  see,  at  once,  that  the  period  of  its 
actual  duration  must  be  very  short  indeed. 

The  exact  duration  of  a  flash  of  lightning  is  a  question  on  which  no 
settled  opinion  has  yet  been  accepted  generally  by  scientific  men. 
Indeed,  the  most  widely  different  statements  have  been  made  on  the 
subject,  quite  recently,  by  the  highest  authorities,  each  speaking  ap- 
parently with  unhesitating  confidence.  Thus,  for  example,  Professor 
Mascart  describes  an  experiment,  which  he  says  was  made  by  Wheat- 
stone,  and  which  showed  that  a  flash  of  lightning  lasts  for  less  than 
0//£-thousandth  of  a  second  ;'  Professor  Everett  describes  the  same 
experiment,  without  saying  by  whom  it  was  made,  and  gives,  as  the 
result,  that  "the  duration  of  the  illumination  produced  by  lightning 
is  certainly  less  than  the  /^//-thousandth  of  a  second;"3  Professor 
Tyndall,  in  his  own  picturesque  way,  tells  us  that  "a  flash  of  light- 
ning cleaves  a  cloud,  appearing  and  disappearing  in  less  than  the 
////W/r</-thousandth  of  a  second  ;  " '  and  according  to  Professor  Tait, 
of  Edinburgh,  "Wheatstone  has  shown  that  lightning  certainty  lasts 
less  than  the  millionth  of  a  second."4 

Experiments  of  Professor  Rood. — I  cannot  say  which  of 
these  statements  is  best  supported  by  actual  observation  ;  for  none  of 
the  writers  I  have  quoted  gives  any  reference  to  the  original  memoir 
from  which  his  statement  is  derived.  As  far  as  my  own  reading 
goes,  I  have  only  come  across  one  original  record  of  experiments, 
made  directly  on  the  flash  of  lightning  itself,  with  a  view  to  determine 
the  period  of  its  duration.  These  experiments  were  carried  out  by 

1  Electricite  Statique,  ii.,  561. 

a  Deschanel's  Natural  Philosophy,  Sixth  Edition,  p.  641, 

3  Fragments  of  Science,  Fifth  Edition,  p.  311. 

4  Lecture  on  Thunderstorms,  Nature,  vol.  xxii.,  p.  341. 


Ll&HTNfNG  AND  THUNDER.  ii 

Professor  Ogden  Rood,  of  Columbia  College,  New  York,  between  the 
years  1870  and  1873,  and  are  recorded  in  the  American  Journal  of 
Science  and  Arts.1 

For  the  description  of  his  apparatus,  and  for  the  details  of  his  ob- 
servations, I  must  refer  you  to  the  memoir  itself  ;  but  I  may  tell  you 
briefly  that  the  results  at  which  he  arrived,  if  they  be  accepted,  must 
lead  to  a  considerable  modification  of  the  views  previously  entertained 
on  the  subject.  In  the  first  place,  he  satisfied  himself  that  what  ap- 
pears to  the  eye  a  single  flash  of  lightning  is  usually,  if  not  always, 
multiple  in  its  character  ;  consisting,  in  fact,  of  a  succession  of  dis- 
tinct flashes,  which  follow  one  another  with  such  rapidity  as  to  make 
a  continuous  inpression  on  the  retina.  Next,  he  proceeded  to  meas- 
ure approximately  the  duration  of  these  several  component  flashes  ; 
and  he  found  that  it  varied  over  a  wide  range,  amounting  sometimes 
to  fully  the  twentieth  of  a  second,  and  being  sometimes  less  than  the 
sixteen-hundredth  of  a  second. 

Wheatstone's  Experiments. — These  results  are  extremely  in- 
teresting ;  but  we  can  hardly  regard  them  as  finally  established,  until 
they  have  been  confirmed  by  other  observers.  I  may  remark,  how- 
ever, that  they  fit  in  very  well  with  the  experiments  made  by  Pro- 
fessor Wheatstone,  many  years  ago,  on  the  duration  of  the  electric 
spark,  which,  as  I  told  you,  is  a  miniature  flash  of  lightning.  In  these 
classical  experiments,  which  leave  nothing  to  be  desired  in  point  of 
accuracy,  Professor  Wheatstone  showed  that  a  spark  taken  directly 
from  a  Leyden  jar,  or  a  spark  taken  from  the  conductor  of  a  power- 
ful electric  machine,  that  is,  just  such  a  spark  as  you  have  seen  here 
to-day,  lasts  for  less  than  the  millionth  of  a  second. 

But  he  also  showed  that  the  duration  of  the  spark  is  greatly  in- 
creased, when  a  resisting  wire  is  introduced  into  the  path  of  the  dis- 
charge. Thus,  for  example,  when  the  discharge  from  a  Leyden  jar  was 
made  to  pass  through  half  a  mile  of  copper  wire,  with  breaks  at  inter- 
vals, t|*e  sparks  that  appeared  at  these  breaks  were  found  to  last  for 
2To~o -Q  °f  a  second.2  Hence  we  should  naturally  expect  that  the  period 
of  illumination  would  be  still  further  increased,  in  the  case  of  a  flash 
of  lightning,  where  the  resistance  interposed  is  enormously  greater 
than  in  either  of  the  experiments  made  by  Wheatstone.3 

Experiment  of  the  Rotating  Disk. — It  would  be  tedious,  on  an 
occasion  like  the  present,  to  enter  into  an  account  of  Wheatstone's 
beautiful  and  ingenious  method  of  investigation,  by  which  the  above 
facts  have  been  established  ;  but  I  will  show  you  a  much  more 
simple  experiment  which  brings  home  very  forcibly  to  the  mind  how 

i  Third  Series,  vol.  v.,  p.  161. 

3  Phil.  Trans.  Royal  Society,  1834,  vol.  cxxv.,  pp.  583-591. 

3  In  experiments  with  a  Leyden  jar,  Feddersen  has  shown  that  the  duration  of  the  discharge  is  in- 
creased, not  only  by  increasing  the  striking  distance,  but  also  by  increasing  the  size  of  the  jar. 
Now,  a  flash  of  lightning  may  be  regarded  as  the  discharge  of  a  Leyden  jar  of  immense  size,  with  an 
enormous  striking  distance  ;  and  therefore  we  should  expe'ct  that  the  duration  of  the  discharge  should 
be  greatly  prolonged.  See  American  Journal  of  Science  and  Arts,  Third  Series,  vol.  i.,  p.  15. 


LIGHTNING  AND  ^HUNDER. 


exceedingly  short  must  be  the  duration  of  the  electric  spark.  Here  is 
a  circular  disk  of  cardboard,  the  outer  part  of  which,  as  you  see,  is  di- 
vided into  sectors,  black  and  white  alternately,  while  the  space  about 
the  centre  is  entirely  white.  The  disk  is  mounted  on  a  stand,  by  means 
of  which  I  can  make  it  rotate  with  great  velocity.  When  it  is  put 
in  rotation,  the  effect  on  the  eye  is  very  striking — the  central  space 
remains  white  as  before,  but  in  the  outer  rim  the  distinction  of  black 
and  white  absolutely  disappears  and  gives  place  to  a  uniform  gray. 
This  color  is  due  to  the  blending  together  of  black  and  white  in  equal 
proportions  ;  the  blending  being  effected,  not  on  the  cardboard  disk, 
but  on  the  retina  of  the  eye. 

I  mentioned  just  now  that  an  impression  made  on  the  retina  lasts 
for  the  tenth  of  a  second  after  the  cause  of  it  has  been  removed. 


CARDBOARD   DISK   AS   SEEN    WHEN   AT   REST. 


SAME   DISK   AS   SEEN   WHEN    IN    RAPID   ROTATION. 


Now,  when  this  disk  is  in  rotation,  the  sectors  follow  one  another  so 
rapidly  that  the  particular  part  of  space  occupied  at  any  moment  by  a 
white  sector  will  be  occupied  by  a  black  sector  within  a  time  much 
less  than  the  tenth  of  a  second.  It  follows  that  the  impression  made 
by  each  white  sector  remains  on  the  retina  until  the  following  black 
sector  comes  into  the  same  position  ;  and,  in  like  manner,  the  impres- 
sion made  by  each  black  sector  remains  until  the  following  white  sec- 
tor takes  up  the  position  of  the  black.  Therefore,  the  impression 
made  -by  the  whole  outer  rim  is  the  impression  of  black  and  white 
combined — that  is,  the  impression  of  gray. 

So  far,  I  dare  say,  the  phenomenon  is  already  familiar  to  you  all. 
But  I  propose  now  to  show  you  the  revolving  disk  illuminated  by  the 


LIGHTNING  AND  THUNDER.  13 

electric  spark  ;  and  you  will  observe  that,  at  the  moment  of  illumina- 
tion, the  black  and  white  sectors  come  out  as  clearly  and  distinctly  as 
if  the  disk  were  standing  still. 

For  the  success  of  this  experiment  it  is  desirable,  not  only  to  have  a 
brilliant  spark  in  order  to  secure  a  good  illumination  of  the  disk,  but 
also  to  have  a  succession  of  such  sparks,  that  you  may  see  the  phe- 
nomenon frequently  repeated,  and  thus  be  able  to  observe  it  at  your 
leisure.  To  attain  these  two  objects,  I  have  made  the  arrangement 
which  is  here  before  you. 

In  front  of  the  disk  is  a  large  and  very  powerful  Leyden  jar.  The 
rod  connected  with  the  inner  coating  rises  well  above  the  mouth  of 
the  jar,  and  ends  in  a  brass  ball  nearly  opposite  the  centre  of  the  disk. 
Connected  with  the  outer  coating  of  the  jar  is  another  rod  which  like- 
wise ends  in  a  brass  ball,  and  which  is  so  adjusted  that  the  distance 
between  the  two  balls  is  about  an  inch.  The  two  rods  are  connected 
respectively  with  the  two  conductors  of  a  Holtz  machine,  so  that, 
when  the  machine  is  worked,  the  jar  is  first  quickly  charged,  and  then 
it  discharges  itself,  with  a  brilliant  spark,  between  the  two  brass  balls. 
Thus,  by  continuing  to  work  the  machine,  we  can  get,  as  long  as  we 
choose,  a  succession  of  sparks  following  one  another  at  short  and 
regular  intervals  right  in  front  of  the  disk. 

Everything  being  now  ready,  and  the  room  partially  darkened,  the 
disk  is  put  in  rapid  rotation  ;  and  you  can  see,  by  the  twilight  that  re- 
mains, the  outer  rim  a  uniform  gray,  and  the  central  space  white. 
But  when  my  assistant  begins  to  turn  the  Holtz  machine,  and  brilliant 
sparks  leap  out  at  intervals,  the  revolving  disk,  illuminated  for  a  mo- 
ment at  each  discharge,  seems  to  be  standing  still, 'and  shows  the 
black  and  white  sectors  distinctly  visible. 

The  reason  of  this  is  clear :  So  brief  is  the  moment  for  which  the 
spark  endures,  that  the  disk,  though  in  rapid  motion,  makes  no  sensi- 
ble advance  during  that  small  fraction  of  time  ;  therefore,  in  the  im- 
age on  the  retina,  the  impression  made  by  the  white  sectors  remains 
distinct  from  the  impression  made  by  the  black,  and  the  eye  sees  the 
disk  as  it  really  is. 

I  may  notice,  in  passing,  a  very  interesting  consideration,  suggested 
by  this  experiment.  A  cannon  ball  is  now  commonly  discharged  with 
a  velocity  of  about  1,600  feet  a  second.  Moving  with  this  velocity  it 
is,  as  you  know,  under  ordinary  circumstances,  altogether  invisible  to 
the  eye.  But  suppose  it  were  illuminated,  in  the  darkness  of  night, 
by  this  electric  spark,  which  lasts,  we  will  say,  for  the  millionth  of  a 
second.  During  the  moment  of  illumination,  the  cannon  ball  moves 
through  the  millionth  part  of  1,600  feet,  which  is  a  little  less  than  the 
fiftieth  of  an  inch.  Practically,  we  may  say  that  the  cannon  ball  does 
not  sensibly  change  its  place  while  the  spark  lasts.  Further,  the  im- 
pression it  makes  on  the  eye,  from  the  position  it  occupies  at  the 


14  LIGHTNING  AND  THUNDER. 

moment  of  illumination,  remains  on  the  retina  for  at  least  the  tenth 
of  a  second.  Therefore,  if  we  are  looking  toward  that  particular  part 
of  space  where  the  cannon  ball  happens  to  be  at  the  moment  the  spark 
passes,  we  must  see  the  cannon  ball  hanging  motionless  in  the  air, 
though  we  know  it  is  traveling  at  the  rate  of  1,600  feet  a  second,  or 
about  1,000  miles  an  hour. 

Brightness  of  a  Flash  of  Lightning.— I  should  like  to  say 
one  word  about  the  brightness  of  a  flash  of  lightning.  Somewhat 
more  than  thirty  years  ago,  Professor  Swan,  of  Edinburgh,  showed 
that  the  eye  requires  a  sensible  time — about  the  tenth  of  a  second — 
to  perceive  the  full  brightness  of  a  luminous  object.  Further,  he 
proved,  by  a  series  of  interesting  experiments,  that  when  a  flash  of 
light  lasts  for  less  than  the  tenth  of  a  second,  its  apparent  brilliancy 
to  the  eye  is  proportional  to  the  time  of  its  duration.1  Now  consider 
the  consequence  of  these  facts  in  reference  to  the  brightness  of  our 
electric  spark.  If  the  spark  lasted  for  the  tenth  of  a  second,  we 
should  perceive  its  full  brightness  ;  if  it  lasted  for  the  tenth  part  of 
that  time,  we  should  see  only  the  tenth  part  of  its  brightness ;  if  it 
lasted  for  the  hundredth  part,  we  should  see  only  the  hundredth  part 
of  its  brightness  ;  and  so  on.  But  we  know,  in  point  of  fact,  that  it 
lasts  for  less  than  the  millionth  of  a  second,  that  is,  less  than  the 
hundred-thousandth  part  of  the  tenth  of  a  second.  Therefore  we  see 
only  the  hundred-thousandth  part  of  its  real  brightness. 

Here  is  a  startling  conclusion,  and  one,  I  may  say,  fully  justified  by 
scientific  evidence.  That  electric  spark,  brilliant  as  it  appears  to  us, 
is  really  a  hundred  thousand  times  as  bright  as  it  seems  to  be.  We 
cannot  speak  with  the  same  precision  of  a  flash  of  lightning  ;  because 
its  duration  has  not  yet  been  so  exactly  determined.  But  if  we  sup- 
pose that  a  flash  of  lightning,  in  a  particular  case,  lasts  for  the  thou- 
sandth of  a  second,  it  would  follow,  from  the  above  experiments,  that 
the  flash  is  a  hundred  times  as  bright,  in  fact,  as  it  appears  to  the  eye. 
•  Various  Forms  of  Lightning. — The  lightning  of  which  I  have 
spoken  hitherto  is  commonly  called  forked  lightning  ;  a  name  which 
seems  to  have  been  derived  from  the  zig-zag  line  of  light  it  presents 
to  the  eye.  But  there  are  other  forms  under  which  the  electricity  of 
the  clouds  often  makes  itself  manifest  ;  and  to  these  I  would  now  in- 
vite your  attention  for  a  few  moments.  The  most  common  of  them 
all,  at  least  in  this  country,  is  that  which  is  familiarly  known  by  the 
name  of  sheet^ lightning.  This  is,  probabJv,  nothing  else  than  the 
lighting  up  of  the  atmosphere,  or  of  the  clouds,  by  forked  lightning, 
which  is  not  itself  directly  visible. 

Generally  speaking,  after  a  flash  of  sheet  lightning,  we  hear  the 
rolling  of  distant  thunder.  But  it  sometimes  happens,  especially  in 

1  See  original  paper  by  Swan,  Trans.  Royal  Society.  Edinburgh.  1849.  vol.  xvi.,  pp.  581-603 ;  also,  a 
second  paper,  ib.  1861,  vol.  xxii.,  pp.  33-39. 


LIGHTNING  AND  THUNDER.  15 

summer  time,  that  the  atmosphere  is  again  and  again  lit  up  by  a  sud- 
den glow  of  light,  and  yet  no  thunder  is  heard.  This  phenomenon  is 
commonly  called  suimner  lightning,  or  heat  lightning.  It  is  probably 
due,  in  many  cases,  to  electrical  discharges  in  the  higher  regions  of 
the  atmosphere,  where  the  air  is  greatly  rarified  ;  and,  in  these  cases,  it 
would  seem  to  resemble  the  discharges  obtained  by  means  of  an  induc- 
tion coil  in  glass  tubes  containing  rarified  gases.  But  there  is  little 
doubt  that  in  many  cases,  too,  summer  lightning,  like  ordinary  sheet 
lightning,  is  due  to  forked  lightning,  which  is  so  remote  that  we  can 
neither  see  the  flash  itself  directly,  nor  hear  the  rolling  of  the  thunder. 

Perhaps  the  most  distinct  and  satisfactory  evidence  on  this  subject, 
derived  from  actual  observation,  is  contained  in  the  following  letter 
of  Professor  Tyndall,  written  in  May,  1883  :  "  Looking  to  the  south 
and  south-east  from  the  Bel  Alp,  the  play  of  silent  lightning  among 
the  clouds  and  mountains  is  sometimes  very  wonderful.  It  may  be 
seen  palpitating  for  hours,  with  a  barely  appreciable  interval  between 
the  thrills.  Most  of  those  who  see  it  regard  it  as  lightning  without 
thunder — Blitz  ohne  Donner,  Wetterleuchten,  I  have  heard  it  named 
by  German  visitors.  The  Monte  Generoso,  overlooking  the  Lake  of 
Lugano,  is  about  fifty  miles  from  the  Bel  Alp,  as  the  crow  flies.  The 
two  points  are  connected  by  telegraph  ;  and  frequently  when  the 
Wetterleuchten,  as  seen  from  the  Bel  Alp,  was  in  full  play,  I  have 
telegraphed  to  the  proprietor  of  the  Monte  Generoso  Hotel  and 
learned,  in  every  instance,  that  our  silent  lightning  co-existed  in  time 
with  a  thunderstorm  more  or  less  terrific  in  upper  Italy."  1 

Another  form  of  lightning,  described  by  many  writers,  is  called 
globg^  lightning.  It  is  said  to  appear  as  a  ball  of  fire,  about  the  size 
of  a  child's  head,  or  even  larger,  which  moves  for  a  time  slowly  about, 
and  then,  after  the  lapse  of  several  seconds,  explodes  with  a  terrific 
noise,  sending  forth  flashes  of  fire  in  all  directions,  which  burn  what- 
ever they  strike.  Many  accounts  are  on  record  of  such  phenomena  ; 
but  they  are  derived,  for  the  most  part,  from  the  evidence  of  persons 
who  were  not  specially  competent  to  observe,  and  to  describe  with 
precision,  the  facts  that  fell  under  their  observation.  Hence  these 
accounts,  while  they  are  accepted  by  some,  are  rejected  by  others  ; 
and  it  seems  to  me,  in  the  present  state  of  the  question,  that  the  ex- 
istence of  globe  lightning  can  hardly  be  regarded  as  a  demonstrated 
fact.  ''At  all  events,  if  phenomena  of  this  kind  have  really  occurred, 
I  can  only  say  that  nothing  we  know  about  electricity,  at  present,  will 
enable  us  to  account  for  them.2 

St.  Elmo's  Fire. — A  much  more  authentic  and,  at  the  same  time, 

1  Nature,  vol.  xxviii.,  p.  54. 

2  See,  however,  an  attempt  to  account  for  this  phenomenon  in  De  Larive's  Treatise  on  Electricity, 
London,  1853-8,  vol.  iii.,  pp.  199,  200  ;  and  another,  quite  recently,  by  Mr.  Spottiswoode,  in  a  Lecture 
on  the  Electrical  Discharge,  delivered  before  the  British  Association  at  York,  in  September,  1881,  and 
published  by  Longmans,  London,  p.  42.    See  also,  for  recent  evidence  regarding  the  phenomenon  itself, 
Scott  s  Elementary  Meteorology,  pp.  175-8. 


16  LIGHTNING  AND  THUNDER. 

very  interesting  form,  under  which  the  electricity  of  the  clouds  some- 
times manifests  its  presence,  is  known  by  the  name  of  St.  Elmo's  fire. 
This  phenomenon  at  one  time  presents  the  appearance  of  a  star,  shining 
at  the  points  of  the  lances  or  bayonets  of  a  company  of  soldiers;  at  an- 
other, it  takes  the  form  of  a  tuft  of  bluish  light,  which  seems  to  stream 
away  from  the  masts  and  spars  of  a  ship  at  sea,  or  from  the  pointed 
spire  of  a  church.  It  was  well  known  to  the  ancients.  Caesar,  in  his 
Commentaries,  tells  us  that,  after  a  stormy  night,  the  iron  points  of 
the  javelins  of  the  fifth  legion  seemed  to  be  on  fire  ;  and  Pliny  says 
that  he  saw  lights,  like  stars,  shining  on  the  lances  of  the  soldiers, 
keeping  watch  by  night  upon  the  ramparts.  When  two  such  lights 
appeared  at  once,  on  the  masts  of  a  ship,  they  were  called  Castor  and 
Pollux,  and  were  regarded  by  sailors  as  a  sign  of  a  prosperous  voyage 
When  only  one  appeared,  it  was  called  Helen,  and  was  taken  as  an 
unfavorable  omen. 

In  modern  times  St.  Elmo's  fire  has  been  witnessed  by  a  host  of 
observers,  and  all  its  various  phases  have  been  repeatedly  described. 
In  the  memoirs  of  Forbin  we  read  that,  when  he  was  sailing  once,  in 
1696,  among  the  Balearic  Islands,  a  sudden  storm  came  on  during  the 
night,  accompanied  by  lightning  and  thunder.  "We  saw  on  the  ves- 
sel," he  says,  "  more  than  thirty  St.  Elmo's  fires.  Among  the  rest 
there  was  one  on  the  vane  of  the  mainmast  more  than  a  foot  and  a 
half  high.  I  sent  a  man  up  to  fetch  it  down.  When  he  was  aloft  he 
cried  out  that  it  made  a  noise  like  wetted  gunpowder  set  on  fire.  1 
told  him  to  take  off  the  vane  and  come  down  ;  but,  scarcely  had  he 
removed  it  from  its  place,  when  the  fire  left  it  and  reappeared  at  the 
end  of  the  mast,  so  that  it  was  impossible  to  take  it  away.  It  remained 
for  a  long  time,  and  gradually  went  out." 

On  the  i4th  of  January,  1824,  Monsieur  Maxadorf  happened  to  look 
at  a  load  of  straw  in  the  middle  of  a  field  just  under  a  dense  black 
cloud.  The  straw  seemed  literally  on  fire — a  streak  of  light  went 
forth  from  every  blade  ;  even  the  driver's  whip  shone  with  a  pale-blue 
flame.  As  the  black  cloud  passed  away,  the  light  gradually  disap- 
peared, after  having  lasted  about  ten  minutes.  Again,  it  is  related 
that  on  the  8th  of  May,  1831,  in  Algiers,  as  the  French  artillery  offi- 
cers were  walking  out  after  sunset  without  their  caps,  each  one  saw  a 
tuft  of  blue  light  on  his  neighbor's  head  ;  and,  when  they  stretched 
out  their  hands,  a  tuft  of  light  was  seen  at  the  end  of  every  finger. 
Not  infrequently  a  traveler  in  the  Alps  sees  the  same  luminous  tuft 
on  the  point  of  his  alpenstock.  And  quite  recently,  during  a  thunder- 
storm, a  whole  forest  was  observed  to  become  luminous  just  before 
each  flash  of  lightning,  and  to  become  dark  again  at  the  moment  of 
the  discharge.1 

»  See  Jamin,  "  Cours  de  Physique,"  i.,  480-1;  Tomlinson,  "  The  Thunderstorm,"  Third  Edition,  pp. 
95-103 ;  "  Thunderstorms,"  a  Lecture  by  Professor  Tail,  Nature,  vol.  xxii.,  p.  356. 


LIGHTNING  AND  THUNDER.  17 

This  phenomenon  may  be  easily  explained.  It  consists  in  a  gradual 
and  comparatively  silent  electrical  discharge  between  the  earth  and 
the  cloud  ;  and  generally,  but  not  always,  it  has  the  effect  of  prevent- 
ing such  an  accumulation  of  electricity  as  would  be  necessary  to  pro- 
duce a  flash  of  lightning.  I  can  illustrate  this  kind  of  discharge  with 
the  aid  of  our  machine.  If  I  hold  a  pointed  metal  rod  toward  the 
large  conductor,  you  can  see,  when  the  machine  is  worked  and  the 
room  darkened,  how  the  point  of  the  rod  becomes  luminous  and  shines 
like  a  faint  blue  star.  I  substitute  for  the  pointed  rod  the  blunt  han- 
dles of  a  pair  of  pliers,  and  a  tuft  of  blue  light  is  at  once  developed  at 
the  end  of  each  handle,  and  seems  to  stream  away  with  a  hissing 
noise.  I  now  put  aside  the  pliers,  and  open  out  my  hand  under  the 
conductor — and  observe  how  I  can  set  up,  at  pleasure,  a  luminous 
tuft  at  the  tips  of  my  fingers.  Now  and  tjien  a  spark  passes,  giving 
me  a  smart  shock,  and  showing  how  the  electricity  may  sometimes 
accumulate  so  fast  that  it  cannot  be  sufficiently  discharged  by  the  lu- 


THE   BRUSH   DISCHARGE,  ILLUSTRATING   ST.  ELMO  S   FIRE. 

minous  tuft.  Lastly,  I  present  a  small  bushy  branch  of  a  tree  to  the 
conductor,  and  all  its  leaves  and  twigs  are  aglow  with  bluish  light, 
which  ceases  for  a  moment  when  a  spark  escapes,  to  be  again  renewed 
when  electricity  is  again  developed  by  the  working  of  the  machine. 

Now,  if  you  put  a  thundercloud  in  the  place  of  that  conductor,  you 
can  easily  realize  how,  through  its  influence,  the  lance  and  bayonet  of 
the  soldier,  the  alpenstock  of  the  traveler,  the  pointed  spire  of  a  church, 
the  masts  of  a  ship  at  sea,  the  trees  of  a  forest,  can  all  be  made  to 
glow  with  a  silent  electrical  discharge  which  may  or  may  not,  accord- 
ing to  circumstances,  culminate  at  intervals  in  a  genuine  flash  of 
lightning. 

Origin  of  Lightning. — When  we  seek  to  account  for  the  origin 
of  lightning,  we  are  confronted  at  once  with  two  questions  of  great 
interest  and  importance — first,  What  are  the  sources  from  which  the 
electricity  of  the  thundercloud  is  derived  ?  and,  secondly,  How  does 
this  electricity  come  to  be  developed  in  a  form  which  so  far  transcends 
in  power  the  electricity  of  our  machines  ?  These  questions  have  long 


1 8  LIGHTNING  AND  THUNDER. 

engaged  the  attention  of  scientific  men,  but  I  cannot  say  that  they 
have  yet  received  a  perfectly  satisfactory  solution.  Nevertheless, 
some  facts  of  great  scientific  value  have  been  established,  and  some 
speculations  have  been  put  forward,  which  are  well  deserving  of  con- 
sideration. 

In  the  first  place,  it  is  quite  certain  that  the  atmosphere  which  sur- 
rounds our  globe  is  almost  always  in  a  state  of  electrification.  Fur- 
ther, the  electrical  condition  of  the  atmosphere  would  seem  to  be  as 
variable  as  the  wind.  It  changes  with  the  change  of  season  ;  it 
changes  from  day  to  day  ;  it  changes  from  hour  to  hour.  The  charge 
of  electricity  is  sometimes  positive,  sometimes  negative  ;  sometimes  it 
is  strong,  sometimes  feeble  ;  and  the  transition  from  one  condition  to 
another  is  sometimes  slow  and  gradual,  sometimes  sudden  and 
violent. 

As  a  general  rule,  in  fine,  clear  weather,  the  electricity  of  the  at- 
mosphere is  positive,  and  not  very  strongly  developed.  In  wet 
weather  the  charge  may  be  either  positive  or  negative,  and  is  gen- 
erally strong,  especially  when  there  are  sudden  heavy  showers.  In 
fog  it  is  also  strong,  and  almost  always  positive.  In  a  snowstorm  it  is 
very  strong,  and  most  frequently  positive.  Finally,  in  a  thunderstorm 
it  is  extremely  strong,  and  generally  negative  ;  but  it  is  subject  to  a 
sudden  change  of  sign,  when  a  flash  of  lightning  passes  or  when  rain 
begins  to  fall. 

So  far  I  have  simply  stated  facts,  which  have  been  ascertained  by 
careful  observations,  made  at  different  stations  by  competent  observ- 
ers, and  extending  over  a  period  of  many  years.  But  as  regards  the 
process  by  which  the  electricity  of  the  atmosphere  is  developed,  we 
have,  up  to  the  present  time,  no  certain  knowledge.  It  has  been  said 
that  electricity  may  be  generated  in  the  atmosphere  by  the  friction  of 
the  air  itself,  and  of  the  minute  particles  floating  in  it,  against  the 
surface  of  the  earth,  against  trees  and  buildings,  against  rocks,  cliffs, 
and  mountains.  But  this  opinion,  however  probable  it  may  be,  has 
not  yet  been  confirmed  by  any  direct  experimental  investigation. 

The  second  theory  is  that  the  electricity  of  the  atmosphere  is  due, 
in  great  part  at  least,  to  the  evaporation  of  salt  water.  Many  years 
ago,  Pouillet,  a  French  philosopher,  made  a  series  of  experiments  in  the 
laboratory,  which  seemed  to  show  that  evaporation  is  generally 
attended  with  the  development  of  electricity  ;  and,  in  particular,  he 
satisfied  himself  that  the  vapor  which  passes  off  from  the  surface  of 
salt  water  is  always  positively  electrified.  Now,  the  atmosphere  is 
everywhere  charged,  more  or  less,  with  vapor  which  comes,  almost 
entirely,  from  the  salt  water  of  the  ocean.  Hence  Pouillet  inferred 
that  the  chief  source  of  atmospheric  electricity  is  the  evaporation  of 
sea  water.  This  explanation  would  certainly  go  far  to  account  for 
the  presence  of  electricity  in  the  atmosphere,  if  the  fact  on  which  it 


LIGHTNING  AND  THUNDER.  19 

i 
rests  were  established  beyond  dispute.     But  there  is  some  reason  to 

doubt  whether  the  development  of  electricity,  in  the  experiments  of 
Pouillet,  was  due  simply  to  the  process  of  evaporation,  and  not  rather 
to  other  causes,  the  influence  of  which  he  did  not  sufficiently  take  into 
account. 

A  conjecture  has  recently  been  started  that  electricity  may  be  gen- 
erated by  the  mere  impact  of  minute  particles  of  water  vapor  against 
minute  particles  of  air.1  If  this  conjecture  could  be  established  as  a 
fact,  it  would  be  amply  sufficient  to  account  for  all  the  electricity  of 
the  atmosphere.  From  the  very  nature  of  a  gas,  the  molecules  of 
which  it  is  composed  are  forever  flying  about  with  incredible  velocity; 
and  therefore  the  particles  of  water  vapor  and  the  particles  of  air, 
which  exist  together  in  the  atmosphere,  must  be  incessantly  coming 
into  collision.  Hence,  however  small  may  be  the  charge  of  electricity 
developed  at  each  individual  impact,  the  total  amount  generated  over 
any  considerable  area,  in  a  single  day,  must  be  very  great  indeed.  It 
is  evident,  however,  that  this  method  of  explaining  the  origin  of  at- 
mospheric electricity  can  only  be  regarded  as,  at  best,  a  probable 
hypothesis,  until  the  assumption  on  which  it  rests  is  supported  by  the 
evidence  of  observation  or  experiment. 

Length  of  a  Flash  of  Lightning. — It  would  seem,  then,  that 
we  are  not  yet  in  a  position  to  indicate  with  certainty  the  sources  from 
which  the  electricity  of  the  atmosphere  is  derived.  But  whatever 
these  sources  may  be,  there  can  be  little  doubt  that  the  electricity  of 
the  atmosphere  is  intimately  associated  with  the  minute  particles  of 
water  vapor  of  which  the  thundercloud  is  eventually  built  up.  This 
consideration  is  of  great  importance  when  we  come  to  consider  the 
special  properties  of  lightning,  as  compared  with  other  forms  of  elec- 
tricity. The  most  striking  characteristic  of  lightning  is  the  wonder- 
ful power  it  possesses  of  forcing  its  way  through  the  resisting  me- 
dium of  the  air.  In  this  respect  it  incomparably  surpasses  all  forms 
of  electricity  that  have  hitherto  been  produced  by  artificial  means. 
The  spark  of  an  ordinary  electric  machine  can  leap  across  a  space  of 
three  or  four  inches  ;  the  machine  we  have  employed  in  our  experi- 
ments to-day  can  give,  under  favorable  circumstances,  a  spark  of  nine 
or  ten  inches ;  the  longest  electric  spark  ever  yet  produced  artificially 
is  probably  the  spark  of  Mr.  Spottiswoode's  gigantic  induction  coil ; 
and  it  does  not  exceed  three  feet  six  inches.  But  the  length  of  a  flash 
of  lightning  is  not  to  be  measured  in  inches,  or  in  feet  or  in  yards  ; 
it  varies  from  one  or  two  miles,  for  ordinary  flashes,  to  eight  or  ten 
miles  in  exceptional  cases. 

This  power  of  discharging  itself  violently  through  a  resisting  me- 
dium, in  which  the  thundercloud  so  far  transcends  the  conductor  of 
an  electric  machine,  is  due  to  the  property  commonly  known  among 

1  Professor  Tait,  On  Thunderstorms,  Nature,  vol.  xxii.,  pp.  436-7. 


26  LIGHTNING  AND  THUNDER. 

scientific  men  as  electrical  potential.  The  greater  the  distance  to 
which  an  electrified  body  can  shoot  its  flashes  through  the  air,  the 
higher  must  be  its  potential.  Hence  the  potential  of  a  thundercloud 
must  be  exceedingly  high,  since  its  flashes  can  pierce  the  air  to  a  dis- 
tance of  several  miles.  And  what  I  want  to  point  out  is,  that  we  are 
able  to  account  for  this  exceedingly  high  potential,  if  we  may  only 
assume  that  the  minute  particles  of  water  vapor  in  the  atmosphere 
have,  from  any  cause,  received  ever  so  small  a  charge  of  electricity. 
The  number  of  such  particles  that  go  to  make  up  an  ordinary  drop  of 
rain  are  to  be  counted  by  millions  of  millions  ;  and  it  is  capable  of 
scientific  proof  that,  as  each  new  particle  is  added,  in  the  building  up 
of  the  drop,  a  rise  of  potential  is  necessarily  produced.  It  is  clear, 
therefore,  that  there  is  practically  no  limit  to  the  potential  that  may 
be  developed  by  the  simple  agglomeration  of  very  small  cloud  parti- 
cles, each  carrying  a  very  small  charge  of  electricity.' 

This  explanation,  which  traces  the  exceedingly  high  potential  of 
lightning  to  the  building  up  of  rain  drops  in  the  thundercloud,  sug- 
gests a  reason  why  it  so  often  happens  that  immediately  after  a  flash 
of  lightning  "the  big  rain  comes  dancing  to  the  earth."  The  poten- 
tial has  been  steadily  rising  as  the  drops  have  been  getting  larger  and 
larger,  until  at  length  the  potential  has  become  so  high  that  the  thun- 
dercloud is  able  to  discharge  itself,  and  almost  at  the  same  moment 
the  drops  have  become  so  large  that  they  can  no  longer  be  held  aloft 
against  the  attracting  force  of  gravity. 

Physical  Cause  of  Thunder. — Let  us  now  proceed  to  consider 
the  phenomenon  of  thunder,  which  is  so  intimately  associated  with 
lightning,  and  which,  though  perfectly  harmless  in  itself,  and  though 
never  heard  until  the  real  danger  is  past,  often  excites  more  terror  in 
the  mind  than  the  lightning  flash  itself.  The  sound  of  thunder,  like 
that  of  the  electric  spark,  is  due  to  a  disturbance  caused  in  the  air  by 
the  electric  discharge.  The  air  is  first  expanded  by  the  intense  heat 
that  is  developed  along  the  line  of  discharge,  and  then  it  rushes  back 
again  to  fill  up  the  partial  vacuum  which  its  expansion  has  produced. 
This  sudden  movement  gives  rise  to  a  series  of  sound  waves,  which 
reach  the  ear  in  the  form  of  thunder.  But  there  are  certain  peculiar 
characteristics  of  thunder  which  are  deserving  of  special  consid- 
eration. 

Rolling  of  Thunder. — They  maybe  classified,  I  think,  under  two 
heads.  First,  the  sound  of  thunder  is  not  an  instantaneous  report 
like  the  sound  of  the  electric  spark — it  is  a  prolonged  peal  lasting, 
sometimes,  for  several  seconds.  Secondly,  each  flash  of  lightning 
gives  rise,  not  to  one  peal  only,  but  to  a  succession(of  peals  following 
one  another  at  irregular  intervals.  These  two  phenomena,  taken 
together,  produce  that  peculiar  effect  on  the  ear  which  is  commonly 

1  See  note  at  the  end  of  this  Lecture,  p.  26. 


LIGHTNING  AND  THUNDER.  *i 

described  as  the  rolling  of  thunder  ;  and  both  of  them,  I  think,  may 
be  sufficiently  accounted  for  in  accordance  with  the  well-established 
properties  of  sound. 

To  understand  why  the  sound  of  thunder  reaches  the  ear  as  a  pro- 
longed peal,  we  have  only  to  remember  that  sound  takes  time  to 
travel.  Since  a  flash  of  lightning  is  practically  instantaneous,  we 
may  assume  that  the  sound  is  produced  at  the  same  moment  all  along 
the  line  of  discharge.  But  the  sound  waves,  setting  out  at  the  same 
moment  from  all  points  along  the  line  of  discharge,  must  reach  the 
ear  in  successive  instants  of  time,  arriving  first  from  that  point  which 
is  nearest  to  the  observer,  and  last  from  that  point  which  is  most  dis- 
tant. Suppose,  for  example,  that  the  nearest  point  of.  the  flash  is  a 
mile  distant  from  the  observer,  and  the  farthest  point  two  miles — the 
sound  will  take  about  five  seconds  to  come  from  the  nearest  point, 
and  about  ten  seconds  to  come  from  the  farthest  point ;  and  more- 
over, in  each  successive  instant  from  the  time  the  first  sound  reaches 
the  ear,  sound  will  continue  to  arrive  from  the  successive  points  be- 
tween. Therefore  the  thunder,  though  instantaneous  in  its  origin, 
will  reach  the  ear  as  a  prolonged  peal  extending  over  a  period  of  five 
seconds. 

Succession  of  Peals. — The  succession  of  peals  produced  by  a 
single  flash  of  lightning  is  due  to  several  causes,  each  one  of  which 
may  contribute  more  or  less,  according  to  circumstances,  toward  the 
general  effect.  First,  if  we  accept  the  results  arrived  at  by  Professor 
Ogden  Rood,  of  Columbia  College,  what  appears  to  the  eye  as  a  sin- 
gle flash  of  lightning,  consists,  in  fact,  as  a  general  rule,  of  a  succes- 
sion of  flashes,  each  one  of  which  must  naturally  produce  its  own  peal 
of  thunder  ;  and  although  the  several  flashes,  if  they  follow  one  an- 
other at  intervals  of  the  tenth  of  a  second,  will  make  one  continuous 
impression  on  the  eye,  the  several  peals  of  thunder,  under  the  same 
conditions,  will  impress  the  ear  as  so  many  distinct  peals. 

The  next  cause  that  I  would  mention  is  the  zigzag  path  of  the 
lightning  discharge.  To  make  clear  to  you  the  influence  of  this  cir- 
cumstance, I  must  ask  your  attention  for  a  moment  to  the  diagram 
on  next  page.  Let  the  broken  line  represent  the  path  of  a  flash  of 
lightning,  and  let  o  represent  the  position  of  an  observer.  The  sound 
will  reach  him  first  from  the  point  A,  which  is  nearest  to  him,  and  then 
it  will  continue  to  arrive  in  successive  instants  from  the  successive 
points  along  the  line  A  N  and  along  the  line  A  M,  thus  producing  the 
effect  of  a  continuous  peal.  Meanwhile  the  sound  waves  have  been 
traveling  from  the  point  B,  and  in  due  time  will  reach  the  observer  at 
o.  Coming  as  they  do  in  a  different  direction  from  the  former,  they 
will  strike  the  ear  as  the  beginning  of  a  new  peal  which,  in  its  turn, 
will  be  prolonged  by  the  sound  waves  arriving,  in  successive  instants, 
from  the  successive  points  along  the  line  B  M  and  B  H.  A  little  later, 


22  LIGHTNING  AND  THUNDER. 

/ 

the  sound  will  arrive  from  the  more  distant  point  c,  and  a  third  peal 
will  begin.  And  so  there  will  be  several  distinct  peals  proceeding,  so 
to  speak,  from  several  distinct  points  in  the  path  of  the  lightning 
flash, 

A  third  cause  to  which  the  succession  of  peals  may  be  referred  is  to 
be  found  in  the  minor  electrical  discharges  that  must  often  take  place 
within  the  thundercloud  itself.  A  thundercloud  is  not  a  continuous 
mass  like  the  metal  cylinder  of  this  electric  machine — it  has  many 
outlying  fragments,  more  or  less  imperfectly  connected  with  the  prin- 
cipal body.  Moreover,  the  material  of  which  the  cloud  is  composed 
is  only  a  very  imperfect  conductor  as  compared  with  our  brass  cylin- 
der. For  these  two  reasons  it  must  often  happen,  about  the  time  a 
flash  of  lightning  passes,  that  different  parts  of  the  cloud  will  be  in 
such  different  electrical  conditions  as  to  give  rise  to  electrical  dis- 
charges within  the  cloud  itself.  Each  of  these  discharges  produces 


ORIGIN   OF   SUCCESSIVE   PEALS  OF  THUNDER. 

its  own  peal  of  thunder  ;  and  thus  we  may  have  a  number  of  minor 
peals,  sometimes  preceding  and  sometimes  following  the  great  crash 
which  is  due  to  the  principal  discharge. 

Lastly,  the  influence  of  echo  has  often  a  considerable  share  in  multi- 
plying the  number  of  peals  of  thunder.  The  waves  of  sound,  going 
forth  in  all  directions,  are  reflected  from  the  surfaces  of  mountains, 
forests,  clouds,  and  buildings,  gnd  coming  back  from  different  quar- 
ters, and  with  varying  intensity,  reach  the  ear  like  the  roar  of  distant 
artillery.  The  striking  effect  of  these  reverberations  in  a  mountain 
district  has  been  described  by  a  great  poet  in  words  which,  I  daresay, 

are  familiar  to  most  of  you  : 

"  Far  along, 
From  peak  to  peak,  the  rattling  crags  among, 

Leaps  the  live  thunder  !  Not  from  one  lone  cloud, 
But  every  mountain  now  has  found  a  tongue, 
And  Jura  answers  from  her  misty  shroud 
Back  to  the  joyous  Alps,  that  call  to  her  aloud  ! " 


LIGHTNING  AND  THUNDER.  23 

Variations  of  Intensity  in  Thunder, — From  what  has  been  said, 
it  is  easy  to  understand  how  the  general  roar  of  thunder  is  subject  to 
great  changes  of  intensity,  during  the  time  it  lasts,  according  to  the 
number  of  peals  that  may  be  arriving  at  the  ear  of  an  observer  in 
each  particular  moment.  But  every  one  must  have  observed  that 
even  an  individual  peal  of  thunder  often  undergoes  similar  changes, 
swelling  out  at  one  moment  with  great  power,  and  the  next  moment 
rapidly  dying  away.  To  account  for  this  phenomenon,  I  would  ob- 
serve, first,  that  there  is  no  reason  to  suppose  that  the  disturbance 
caused  by  lightning  is  of  exactly  the  same  magnitude  at  every  point 
of  its  path.  On  the  contrary,  it  would  seem  very  probable  that  the 
amount  of  this  disturbance  is,  in  some  way,  dependent  on  the  resist- 
ance which  the  discharge  encounters.  Hence  the  intensity  of  the 
sound  waves  sent  forth  by  a  flash  of  lightning  is  probably  very  differ- 
ent at  different  parts  of  its  course  ;  and  each  individual  peal  will 
swell  out  on  the  ear  or  die  away,  according  to  the  greater  or  less  in- 
tensity of  the  sound  waves  that  reach  the  ear  in  each  successive 
moment  of  time. 

But  there  is  another  influence  at  work  which  must  produce  varia- 
tions in  the  loudness  of  a  peal  of  thunder,  even  though  the  sound 
waves,  set  in  motion  by  the  lightning,  were  everywhere  of  equal  in- 
tensity. This  influence  depends  on  the  position  of  the  observer  in 
relation  to  the  path  of  the  lightning  flash.  At  one  part  of  its  course 
the  lightning  may  follow  a  path  which  remains  for  a  certain  length 
at  nearly  the  same  distance  from  the  observer  ;  then  all  the  sound 
produced  along  this  length  will  reach  the  observer  nearly  at  the  same 
moment,  and  will  burst  upon  the  ear  with  great  intensity.  At  another 
part,  the  lightning  may  for  an  equal  length  go  right  away  from  the 
observer  ;  and  it  is  evident  that  the  sound  produced  along  this  length 
will  reach  the  observer  in  successive  instants,  and  consequently  pro- 
duce an  effect  comparatively  feeble. 

With  a  view  to  investigate  this  interesting  question  a  little  more 
closely,  let  me  suppose  the  position  of  the  observer  taken  as  a  centre, 
and  a  number  of  concentric  circles  drawn,  cutting  the  path  of  the 
lightning  flash,  and  separated  from  one  another  by  a  distance  of  no 
feet,  measured  along  the  direction  of  the  radius.  It  is  evident  that 
all  the  sound  produced  between  any  two  consecutive  circles  will  reach 
the  ear  within  a  period  which  must  be  measured  by  the  time  that 
sound  takes  to  travel  no  feet,  that  is,  within  the  tenth  of  a  second. 
Hence,  in  order  to  determine  the  quantity  of  sound  that  reaches  the 
ear  in  successive  periods  of  one-tenth  of  a  second,  we  have  only  to 
observe  how  much  is  produced  between  each  two  consecutive  cir- 
cles. But  on  the  supposition  that  the  sound  waves,  set  in  motion  by 
the  flash  of  lightning,  are  of  equal  intensity  at  every  point  of  its 
path,  it  is  clear  that  the  quantity  of  sound  developed  between  each 


24  LIGHTNING  AND   THUNDER. 

two  consecutive  circles  will  be  simply  proportional  to  the  length  of 
the  path  enclosed  between  them. 

With  these  principles  established,  let  us  now  follow  the  course  of  a 
peal  of  thunder,  in  the  diagram  before  us.  This  broken  line,  drawn 
almost  at  random,  represents  the  path  of  a  flash  of  lightning  ;  the 
observer  is  supposed  to  be  placed  at  o,  which  is  the  centre  of  the  con- 
centric circles  ;  these  circles  are  separated  from  one  another  by  a  dis- 
tance of  no  feet,  measured  in  the  direction  of  the  radius;  and  we 
want  to  consider  how  any  one  peal  of  thunder  may  vary  in  loudness 
in  the  successive  periods  of  one-tenth  of  a  second. 

Let  us  take,  for  example,  the  peal  which  begins  when  the  sound 
waves  reach  the  ear  from  the  point  A.  In  the  first  unit  of  time  the 
sound  that  reaches  the  ear  is  the  sound  produced  along  the  lines  A  B 
and  A  c  ;  in  the  second  unit,  the  sound  produced  along  the  lines  B  D 
and  c  E  ;  in  the  third  unit,  the  sound  produced  along  D  F  and  E  G. 
So  far  the  peal  has  been  fairly  uniform  in  its  intensity  ;  though  there 
has  been  a  slight  falling  off  in  the  second  and  third  units  of  time,  as 


VARIATIONS   OF   INTENSITY    IN    A    PEAL   OF   THUNDER. 

compared  with  the  first.  But  in  the  fourth  unit  there  is  a  consider- 
able falling  away  of  the  sound  ;  for  the  line  F  K  is  only  about  one- 
third  as  long  as  D  F  and  E  G  taken  together ;  therefore  the  quantity 
of  sound  that  reaches  the  ear  in  the  fourth  unit  of  time  is  only  one- 
third  of  that  which  reaches  it  in  each  of  the  three  preceding  units  ; 
and  consequently  the  sound  is  only  one-third  as  loud.  In  the  fifth 
unit,  however,  the  peal  must  rise  to  a  sudden  crash  ;  for  the  portion 
of  the  lightning  path  inclosed  between  the  fifth  and  sixth  circles  is 
about  six  times  as  great  as  that  between  the  fourth  and  fifth  ;  there- 
fore the  intensity  of  the  sound  will  be  suddenly  increased  about  six- 
fold. After  this  sudden  crash,  the  sound  as  suddenly  dies  away  in 
the  sixth  unit  of  time  ;  it  continues  feeble  as  the  path  of  the  light- 
ning goes  nearly  straight  away  from  the  observer ;  it  swells  again 
slightly  in  the  ninth  unit  of  time  ;  and  then  continues  without  much 
variation  to  the  end.  This  is  only  a  single  illustration,  but  it  seems 
quite  sufficient  to  show  that  the  changes  of  intensity  in  a  peal  of 


LIGHTNING  AND   THUNDER.  25 

thunder  must  be  largely  due  to  the  position  of  the  spectator  in  rela- 
tion to  the  several  parts  of  the  lightning  flash. 

Distance  of  a  Flash  of  Lightning. — I  need  hardly  remind 
you  that,  by  observing  the  interval  that  elapses  between  the  flash  of 
lightning  and  the  peal  of  thunder  that  follows  it,  we  may  estimate 
approximately  the  distance  of  the  nearest  point  of  the  discharge. 
Light  travels  with  such  amazing  velocity  that  we  may  assume,  with- 
out any  sensible  error,  that  we  see  the  flash  of  lightning  at  the  very 
moment  in  which  the  discharge  takes  place.  But  sound,  as  we  have 
seen,  takes  a  sensible  time  to  travel  even  short  distances  ;  and  there- 
fore a  measurable  interval  almost  always  elapses  between  the  moment 
in  which  the  flash  is  seen  and  the  moment  in  which  the  peal  of  thunder 
first  reaches  the  ear.  And  the  distance  through  which  sound  travels 
in  this  interval  will  be  the  distance  of  the  nearest  point  through  which 
the  discharge  has  passed.  Now,  the  velocity  of  sound  in  air  varies 
slightly  with  the  temperature  ;  but,  at  the  ordinary  temperature  of 
our  climate,  we  shall  not  be  far  astray  if  we  allow  1,100  feet  for  every 
second,  or  about  one  mile  for  every  five  seconds. 

You  will  observe  also  that,  by  repeating  this  observation,  we  can 
determine  whether  the  thundercloud  is  coming  toward  us,  or  going 
away  from  us.  So  long  as  the  interval  between  each  successive  flash 
and  the  corresponding  peal  of  thunder,  continues  to  get  shorter  and 
shorter,  the  thundercloud  is  approaching  ;  when  the  interval  begins 
to  increase,  the  thundercloud  is  receding  from  us,  and  the  danger  is 
passed. 

The  crash  of  thunder  is  terrific  when  the  lightning  is  close  at  hand ; 
but  it  is  a  curious  fact,  that  the  sound  does  not  seem  to  travel  as  far 
as  the  report  of  an  ordinary  cannon.  We  have  no  authentic  record  of 
thunder  having  been  heard  at  a  greater  distance  than  from  twelve  to 
fifteen  miles,  whereas  the  report  of  a  single  cannon  has  been  heard  at 
five  times  that  distance  ;  and  the  roar  of  artillery,  in  battle,  at  a 
greater  distance  still.  On  the  occasion  of  the  Queen's  visit  to  Cher- 
bourg, in  August,  1858,  the  salute  fired  in  honor  of  her  arrival  was 
heard  at  Bonchurch,  in  the  Isle  of  Wight,  a  distance  of  sixty  miles. 
It  was  also  heard  at  Lyme  Regis,  in  Dorsetshire,  which  is  eighty-five 
miles  from  Cherbourg,  as  the  crow  flies  ;  and  we  are  told  that,  not 
only  was  it  audible  in  its  general  effect,  but  the  report  of  individual 
guns  was  distinctly  recognized.  The  artillery  of  Waterloo  is  said  to 
have  been  heard  at  the  town  of  Creil,  in  France,  115  miles  from  the 
field  of  battle  ;  and  the  cannonading  at  the  siege  of  Valenciennes,  in 
1793,  was  heard,  from  day  to  day,  at  Deal,  on  the  coast  of  England, 
a  distance  of  120  miles.1 

So  far,  I  have  endeavored  to  set  forth  some  general  ideas  on  the 
nature  and  origin  of  lightning,  and  of  the  thunder  that  accompanies 

1  See  Tomlinson,  The  Thunderstprm,  pp.  87-9. 


26  LIGHTNING  AND  THUNDER. 

it.  In  my  next  Lecture  I  propose  to  give  a  short  account  of  the  de- 
structive effects  of  lightning,  and  to  consider  how  these  effects  may 
best  be  averted  by  means  of  lightning  conductors. 


NOTE  TO  PAGE  20. 

ON  THE  HIGH  POTENTIAL  OF  A  FLASH  OF  LIGHTNING. 

The  potential  of  an  electrified  sphere  is  equal  to  the  quantity  of  electricity  with 
which  the  sphere  is  charged,  divided  by  the  radius  of  the  sphere.  Now  the  minute 
cloud  particles,  which  go  to  make  up  a  drop  of  rain,  may  be  taken  to  be  very  small 
spheres  ;  and  if  v  represent  the  potential  of  each  one,  q  the  quantity  of  electricity  with 
which  it  is  charged,  and  r  the  radius  of  the  sphere,  we  have  v  =—  £  Suppose  1,000 
of  these  cloud  particles  to  unite  into  one  ;  the  quantity  of  electricity  in  the  drop,  thus 
formed,  will  be  i.ooo  q  ;  and  the  radius,  which  increases  in  the  ratio  of  the  cube  root 
of  the  volume,  will  be  lor.  Therefore  the  potential  of  the  new  sphere  will  be 
1000 ?,  or  100  ^  ;  that  is  to  say,  it  will  be  100  times  as  great  as  the  potential  of  each 
of  the  cloud  particles  which  compose  it.  When  a  million  of  cloud  particles  are  blended 
into  a  single  drop,  the  same  process  will  show  that  the  potential  has  been  increased 
ten  thousandfold  ;  and  when  a  drop  is  produced  by  the  agglomeration  of  a  million  of 
millions  of  cloud  particles,  the  potential  of  the  drop  will  be  a  hundred  million  times 
as  great  as  that  of  the  individual  particles.1 


LECTURE  II. 


LIGHTNING    CONDUCTORS. 

THE  effects  of  lightning,  on  the  bodies  that  it  strikes,  are  analo- 
gous to  those  which  may  be  produced  by  the  discharge  of  our 
electric  machines  and  Leyden  jar  batteries.  When  the  discharge  of 
a  battery  traverses  a  metal  conductor  of  sufficient  dimensions  to  allow 
it  an  easy  passage,  it  makes  its  way  along  silently  and  harmlessly. 
But  if  the  conductor  be  so  thin  as  to  offer  considerable  resistance, 
then  the  conductor  itself  is  raised  to  intense  heat,  and  may  be  melted, 
or  even  converted  into  vapor,  by  the  discharge. 

On  opposite  page  is  shown  a  board  on  which  a  number  of  very  thin 
wires  have  been  stretched,  over  white  paper,  between  brass  balls.  The 
wires  are  so  thin  that  the  full  charge  of  the  battery  before  you,  which 
consists  of  nine  large  Leyden  jars,  is  quite  sufficient  to  convert  them  in 
an  instant  into  vapor.  I  have  already,  on  former  occasions,  sent  the 
charge  through  two  of  these  wires,  and  nothing  remains  of  them  now 

1  See  Tait  on  Thunderstorms,  Nature,  vol.  xxii.,  p.  436. 


LIGHTNING  AND  THUNDER. 


27 


but  the  traces  of  their  vapor,  which  mark  the  path  of  the  electric  dis- 
charge from  ball  to  ball.  At  the  present  moment  the  battery  stands 
ready  charged,  and  I  am  going  to  discharge  it  through  a  third  wire,  by 
means  of  this  insulated  rod  which  I  hold  in  my  hand.  The  discharge 


DISCHARGE   OF   LEYDEN   JAR   BATTERY   THROUGH   THIN   WIRES. 

has  passed  ;  you  saw  a  flash,  and  a  little  smoke  ;  and  now,  if  you  look 
at  the  paper,  you  will  find  that  the  wire  is  gone,  but  that  it  has  left 
behind  the  track  of  its  incandescent  vapor,  marking  the  path  of  the 
discharge. 

Destruction  of  Buildings  by  Lightning.— We  learn  from  this 
experiment  that  the  electricity  stored  up  in  our  battery  passes,  with- 
out visible  effect,  through  the  stout  wire  of  a  discharging  rod,  but 
that  it  instantly  converts  into  vapor  the  thin  wire  stretched  across  the 
spark  board.  And  so  it  is  with  a  flash  of  lightning.  It  passes  harm- 
lessly, as  every  one  knows,  through  a  stout  metal  rod,  but  when  it 
comes  across  bell  wires  or  telegraph  wires,  it  melts  them,  or  converts 
them  into  vapor.  On  the  sixteenth  of  July,  1759,  a  flash  of  lightning 
struck  a  house  in  Southwark,  on  the  south  side  of  London,  and  fol- 
lowed the  line  of  the  bell  wire.  After  the  lightning  had  passed,  the 
wire  was  no  longer  to  be  found  ;  but  the  path  of  the  lightning  was 
clearly  marked  by  patches  of  vapor  which  were  left,  here  and  there, 
adhering  to  the  surface  of  the  wall:  In  the  year  1754,  the  lightning 
fell  on  a  bell  tower  at  Newbury,  in  the  United  States  of  America, 
and  having  dashed  the  roof  to  pieces,  and  scattered  the  fragments 
about,  it  reached  the  bell.  From  this  point  it  followed  an  iron  wire, 
about  as  thick  as  a  knitting  needle,  melting  it  as  it  passed  along,  leav- 
ing behind  a  black  streak  of  vapor  on  the  surface  of  the  walls. 

Again,  the  electric  discharge,  passing  through  a  bad  conductor, 
produces  mechanical  disturbance,  and,  if  the  substance  be  combus- 
tible, often  sets  it  on  fire.  So,  too,  as  you  know,  the  lightning  flash, 
falling  on  a  church  spire,  dashes  it  to  pieces,  knocking  the  stones 
about  in  all  directions,  while  it  sets  fire  to  ships  and  wooden  buildings; 
and  more  than  once  it  has  caused  great  devastation  by  exploding 
powder  magazines, 


28  LIGHTNING  AND  THUNDER. 

Let  me  give  you  one  or  two  examples  :  In  January,  1762,  the  light- 
ning fell  on  a  church  tower  in  Cornwall,  and  a  stone — three  hundred- 
weight— was  torn  from  its  place  and  hurled  to  a  distance  of  180  feet, 
while  a  smaller  stone  was  projected  as  far  as  1,200  feet  from  the 
building.  Again,  in  1809,  the  lightning  struck  a  house  not  far  from 
Manchester,  and  literally  moved  a  massive  wall  twelve  feet  high  and 
three  thick  to  a  distance  of  several  feet.  You  may  form  some  con- 
ception of  the  enormous  force  here  brought  into  action,  when  I  tell 
you  that  the  total  weight  of  mason-work  moved  on  this  occasion  was 
not  less  than  twenty-three  tons. 

The  church  of  St.  George,  at  Leicester,  was  severely  damaged  by 
lightning  on  the  ist  of  August,  1846.  About  8  o'clock  in  the  evening 
the  rector  of  the  parish  saw  a  vivid  streak  of  light  darting  with  incred- 
ible velocity  against  the  upper  part  of  the  spire.  "  For  the  distance  of 
forty  feet  on  the  eastern  side,  and  nearly  seventy  on  the  west,  the 
massive  stonework  of  the  spire  was  instantly  rent  asunder  and  laid  in 
ruins.  Large  blocks  of  stone  were  hurled  in  all  directions,  broken 
into  small  fragments,  and  in  some  cases,  there  is  reason  to  believe, 
reduced  to  powder.  One  fragment  of  considerable  size  was  hurled 
against  the  window  of  a  house  three  hundred  feet  distant,  shattering 
to  pieces  the  woodwork,  and  strewing  the  room  within  with  fine  dust 
and  fragments  of  glass.  It  has  been  computed  that  a  hundred  tons 
of  stone  were,  on  this  occasion,  blown  to  a  distance  of  thirty  feet  in 
three  seconds.  In  addition  to  the  shivering  of  the  spire,  the  pinnacles 
at  the  angles  of  the  tower  were  all  more  or  less  damaged,  the  flying 
buttresses  cracked  through  and  violently  shaken,  many  of  the  open 
battlements  at  the  base  of  the  spire  knocked  away,  the  roof  of  the 
church  completely  riddled,  the  roofs  of  the  side  entrances  destroyed, 
and  the  stone  staircases  of  the  gallery  shattered."1 

Lightning  has  been  at  all  times  the  cause  of  great  damage  to  prop- 
erty by  its  power  of  setting  fire  to  whatever  is  combustible.  Fuller 
says,  in  his  Church  History,  that  "scarcely  a  great  abbey  exists  in 
England  which  once,  at  least,  has  not  been  burned  by  lightning  from 
heaven."  He  mentions,  as  examples,  the  Abbey  of  Croylarid  twice 
burned,  the  Monastery  of  Canterbury  twice,  the  Abbey  of  Peterbor- 
ough twice  ;  also  the  Abbey  of  St.  Mary's,  in  Yorkshire,  the  Abbey  of 
Norwich,  and  several  others.  Sir  William  Snow  Harris,  writing  about 
twenty  years  ago,  tells  us  that  "  the  number  of  churches  and  church 
spires  wholly  or  partially  destroyed  by  lightning  is  beyond  all  belief, 
and  would  be  too  tedious  a  detail  to  enter  upon.  Within  a  compara- 
tively few  years,  in  1822  for  instance,  we  find  the  magnificent  Cathedral 
of  Rouen  burned,  and,  so  lately  as  1850,  the  beautiful  Cathedral  of 
Saragossa,  in  Spain,  struck  by  lightning  during  divine  service  and  set 
on  fire.  In  March  of  last  year  a  dispatch  from  our  Minister  at  Brussels, 

>  The  Thunderstorm,  by  Charles  Tomlinson,  F.  R.  S.,  Third  Edition,  pp.  153-4. 


LIGHTNING  AND  THUNDER.  29 

Lord  Howard  de  Walden,  dated  the  24th  of  February,  was  forwarded 
by  Lord  Russell  to  the  Royal  Society,  stating  that,  on  the  preceding 
Sunday,  a  violent  thunderstorm  had  spread  over  Belgium  ;  that  twelve 
churches  had  been  struck  by  lightning  ;  and  that  three  of  these  fine 
old  buildings  had  been  totally  destroyed."1 

Even  in  our  own  day  the  destruction  caused  by  fires  produced 
through  the  agency  of  lightning  is  very  great — far  greater  than  is 
commonly  supposed.  No  general  record  of  such  fires  is  kept,  and 
consequently  our  information  on  the  subject  is  very  incomplete  and 
inexact.  I  may  tell  you,  however,  one  small  fact  which,  so  far  as  it 
goes,  is  precise  enough  and  very  significant.  In  the  little  province  of 
Schleswig-Holstein,  which  occupies  an  area  less  than  one-fourth  of 
the  area  of  Ireland,  the  Provincial  Fire  Assurance  Association  has 
paid  in  sixteen  years,  for  damage  caused  by  lightning,  somewhat  over 
^100,000,  or  at  the  rate  of  more  than  ^"6,000  a  year.  The  total  loss 
of  property  every  year  in  this  province,  due  to  fires  caused  by  light- 
ning, is  estimated  at  not  less  than  ^"i2,5oo.2 

Destruction  of  Ships  at  Sea. — The  destructive  effects  of  light- 
ning on  ships  at  sea,  before  the  general  adoption  of  lightning  con- 
ductors, seems  almost  incredible  at  the  present  day.  From  official 
records  it  appears  that  the  damage  done  to  the  Royal  Navy  of  Eng- 
land alone  involved  an  expenditure  of  from  ^"6,000  to  ,£10,000  a 
year.  We  are  told  by  Sir  William  Snow  Harris,  who  devoted  himself 
for  many  years  to  this  subject  with  extraordinary  zeal  and  complete 
success,  that  between  the  year  1810  and  the  year  1815 — that  is,  within 
a  period  of  five  years — "  no  less  than  forty  sail  of  the  line,  twenty 
frigates,  and  twelve  sloops  and  corvettes  were  placed  hors  de  combat 
by  lightning.  In  the  merchant  navy,  within  a  comparatively  small 
number  of  years,  no  less  than  thirty-four  ships,  most  of  them  large 
vessels  with  rich  cargoes,  have  been  totally  destroyed — been  either 
burned  or  sunk — to  say  nothing  of  a  host  of  vessels  partially  destroyed 
or  severely  damaged."3. 

And  these  statements,  be  it  observed,  take  no  account  of  ships  that 
were  simply  reported  as  missing,  some  of  which,  we  can  hardly  doubt, 
were  struck  by  lightning  in  the  open  sea,  and  went  down  with  all 
hands  on  board.  A  famous  ship  of  forty-four  guns,  the  Resistance, 
was  struck  by  lightning  in  the  Straits  of  Malacca,  and  the  powder 
magazine  exploding,  she  went  to  the  bottom.  Of  her  whole  crew  only 
three  were  saved,  who  happened  to  be  picked  up  by  a  passing  boat. 
It  has  been  well  observed  that,  were  it  not  for  these  three  chance  sur- 
vivors, nothing  would  have  been  known  concerning  the  fate  of  the 
vessel,  and  she  would  have  been  simply  recorded  as  missing  in  the 
Admiralty  lists. 

1  Two  Lectures  on  Atmospheric  Electricity  and  Protection  from  Lightning,  published  at  the  end  of 
his  Treatise  on  Frictional  Electricity,  p.  273. 

3  See  Report  of  Lightning  Rod  Conference,  p.  119.  8  Loco  citato. 


30  LIGHTNING  AND  THUNDER. 

Nothing  is  more  fearful  to  contemplate  than  the  scene  on  board  a 
ship  when  she  is  struck  by  lightning  in  the  open  sea,  with  the  winds 
howling  around,  the  waves  rolling  mountains  high,  the  rain  coming 
down  in  torrents,  and  the  vivid  flashes  lighting  up  the  gloom  at  inter- 
vals, and  carrying  death  and  destruction  in  their  track.  I  will  read 
you  one  or  two  brief  accounts  of  such  a  scene,  given  in  the  pithy  but 
expressive  language  of  the  sailor.  In  January,  1786,  the  Thisbe,  of 
thirty-six  guns,  was  struck  by  lightning  off  the  coast  of  Scilly,  and 
reduced  to  the  condition  of  a  wreck.  Here  is  an  extract  from  the 
ship's  log  :  "  Four  A.  M.,  strong  gales ;  handed  mainsail  and  main  top- 
sail ;  hove  to  with  storm  staysails;  blowing  very  heavy,  S.  E.  4.15, 
a  flash  of  lightning,  with  tremendous  thunder,  disabled  some  of  our 
people.  A  second  flash  set  the  mainsail,  main-top,  and  mizen  stay- 
sails on  fire.  Obliged  to  cut  away  the  mainmast ;  this  carried  away 
mizen  top-mast  and  fore  top-sail  yard.  Found  foremast  also  shivered 
by  the  lightning.  Fore  top-mast  went  over  the  side  about  9  A.  M. 
Set  the  foresail."1 

A  few  years  later,  in  March,  1796,  the  Lowestoffe  was  struck  in  the 
Mediterranean,  and  we  read  as  follows  in  the  log  of  the  ship  :  "  North 
end  of  Minorca  ;  heavy  squalls  ;  hail,  rain,  thunder,  and  lightning. 
12.15,  ship  struck  by  lightning,  which  knocked  three  men  from  the  mast- 
head, one  killed.  12.30,  ship  again  struck;  main  top-mast  shivered  in 
pieces  ;  many  men  struck  senseless  on  the  decks.  Ship  again  struck, 
and  set  on  fire  in  the  masts  and  rigging  ;  mainmast  shivered  in  pieces  ; 
fore  top-mast  shivered  ;  men  benumbed  on  the  decks,  and  knocked 
out  of  the  top  ;  one  man  killed  on  the  spot.  1.30,  cut  away  the  main- 
mast;  employed  clearing  wreck.  4,  moderate;  set  the  foresail."8 

Again,  in  1810,  the  Repulse,  a  ship  of  seventy-four  guns,  was  struck, 
off  the  coast  of  Spain.  "The  wind  had  been  variable  in  the  morning- 
and  at  12.35  there  was  a  heavy  squall,  with  rain,  thunder,  and  light- 
ning. The  ship  was  struck  by  two  vivid  flashes  of  lightning,  which 
shivered  the  maintop-gallant  mast,  and  severely  damaged  the  main- 
mast. Seven  men  were  killed  on  the  spot ;  three  others  only  survived 
a  few  days  ;  and  ten  others  were  maimed  for  life.  After  the  second 
discharge  the  rain  fell  in  torrents.  The  ship  was  more  completely 
crippled  than  if  she  had  been  in  action,  and  the  squadron,  then  en- 
gaged on  a  critical  service,  lost  for  a  time  one  of  its  fastest  and  best 
ships."1 

Destruction  of  Powder  Magazines.— Not  less  appalling  is 
the  devastation  caused  by  lightning  when  it  falls  on  a  powder  maga- 
zine. Here  is  a  striking  example  :  On  the  eighteenth  of  August, 
1769,  the  tower  of  St.  Nazaire,  at  Brescia,  was  struck  by  lightning. 
Underneath  the  tower  about  200,000  pounds  of  gunpowder,  belonging 

1  Sir  William  Snow  Harris,  loco  citato^  p.  274. 

3  Id.,  p.  275. 

*  The  Thunderstorm,  by  Charles  Toralinson,  F.R.S.,  Third  Edition,  p.  172. 


LIGHTNING  AND  THUNDER.  31 

to  the  Republic  of  Venice,  were  stored  in  vaults.  .  The  powder  ex- 
ploded, leveling  to  the  ground  a  great  part  of  the  beautiful  city  of 
Brescia,  and  burying  thousands  of  its  inhabitants  in  the  ruins.  It  is 
said  that  the  tower  itself  was  blown  up  bodily  to  a  great  height  in 
the  air,  and  came  down  in  a  shower  of  stones.  This  is,  perhaps,  the 
most  fearful  disaster  of  the  kind  on  record.  But  we  are  not  without 
examples  in  our  own  times.  In  the  year  1856  the  lightning  fell  on  the 
Church  of  St.  John,  in  the  Island  of  Rhodes.  A  large  quantity  of 
gunpowder  had  been  deposited  in  the  vaults  of  the  church.  This  was 
ignited  by  the  flash  ;  the  building  was  reduced  to  a  mass  of  ruins,  a 
large  portion  of  the  town  was  destroyed,  and  a  considerable  number 
of  the  inhabitants  were  killed.  Again,  in  the  following  year,  the 
magazine  of  Joudpore,  in  the  Bombay  Presidency,  was  struck  by 
lightning.  Many  thousand  pounds  of  gunpowder  were  blown  up,  five 
hundred  houses  were  destroyed,  and  nearly  a  thousand  people  are 
said  to  have  been  killed.1 

Experimental  Illustrations. — And  now,  before  proceeding  fur- 
ther, I  will  make  one  or  two  experiments,  with  a  view  of  showing  that 
the  electricity  of  our  machines  is  capable  of  producing  effects  similar 
to  those  produced  by  lightning,  though  immeasurably  inferior  in  point 
of  magnitude.  Here  is  a  common  tumbler,  about  three-quarters  full 
of  water.  Into  it  I  introduce  two  bent  rods  of  brass,  which  are  care- 
fully insulated  below  the  surface  of  the  water  by  a  covering  of  india- 
rubber.  The  points,  however,  are  exposed,  and  come  to  within  an  inch 
of  one  another,  near  the  bottom  of.  the  tumbler.  Outside  the  tum- 
bler, the  brass  rods  are  mounted  on  a  stand,  by  means  of  which  I  can 
send  the  full  charge  of  this  Leyden  jar  battery  through  the  water, 
from  point  to  point.  Since  water  is  a  bad  conductor  of  electricity, 
as  compared  with  metals,  the  charge  encounters  great  resistance  in 
passing  through  it,  and  in  overcoming  this  resistance  produces  con- 
siderable mechanical  commotion,  which  is  usually  sufficient  to  shiver 
the  glass  to  pieces. 

To  charge  the  battery  will  take  about  twenty  turns  of  this  large 
Holtz  machine.  Observe  how  the  pith  ball  of  the  electroscope  rises 
as  the  machine  is  worked,  showing 'that  the  charge  is  going  in.  And 
now  it  remains  stationary  ;  which  is  a  sign  that  the  battery  is  fully 
charged,  and  can  receive  no  more.  You  will  notice  that  the  outside 
coating  of  the  battery  has  been  already  connected  with  one  of  the 
brass  rods  dipping  into  the  tumbler  of  water.  By  means  of  this  dis- 
charger I  will  now  bring  the  inside  coating  into  connection  with  the 
other  rod.  And  see,  before  contact  is  actually  made,  the  spark  has 
leaped  across,  and  our  tumbler  is  violently  burst  asunder  from  top  to 
bottom. 

1  See  for  these  facts,  Anderson,  Lightning  Conductors,  p.  197 ;  1'omlinson,  The  Thunderstorm,  pp. 
167-9  ;  Harris,  loco  citato,  pp.  273-4. 


3*  LIGHTNING  AND  THUNDER. 

This  will  probably  appear  to  you  a  very  small  affair,  when  com- 
pared with  the  tearing  asunder  of  solid  masonry,  and  the  hurling 
about  of  stones  by  the  ton  weight.  No  doubt  it  is  ;  and  that  is  just 
one  of  the  lessons  we  have  to  learn  from  the  experiment  we  have 
made.  For,  not  only  does  it  show  us  that  effects  of  this  kind  may  be 
caused  by  electricity  artificially  produced,  but  it  brings  home  forcibly 
to  the  mind  how  incomparably  more  powerful  is  the  lightning  of  the 
clouds  than  the  electricity  of  our  machines. 

The  property  which  electricity  has  of  setting  fire  to  combustible 
substances  may  be  easily  illustrated.  This  india  rubber  tube  is  con- 
nected with  the  gas  pipe  under  the  floor,  and  to  the  end  of  the  tube  is 
fitted  a  brass  stop-cock  which  I  hold  in  my  hand.  I  open  the  cock, 


GLASS   VESSEL    BROKEN    MY    DISCHARGE    OF    LEYUEN    JAK    BATTKKY. 

and  allow  the  jet  of  gas  to  flow  toward  the  conductor  of  Carry's  ma- 
chine, while  my  assistant  turns  the  handle  ;  a  spark  passes,  and  the 
gas  is  lit.  Again,  my  assistant  stands  on  this  insulating  stool,  placing 
his  hand  on  the  large  conductor  of  the  machine,  while  I  turn  the  han- 
dle. His  body  becomes  electrified,  and  when  he  presents  his  knuckle 
to  this  vessel  of  spirits  of  wine,  which  is  electrically  connected  with 
the  earth,  a  spark  leaps  across,  and  the  spirits  of  wine  are  at  once  in 
a  blaze.  Once  more  ;  I  tie  a  little  gun-cotton  around  one  knob  of 
the  discharging  rod,  and  then  use  it  to  discharge  a  small  Leyden  jar  ; 
at  the  moment  of  the  discharge  the  gun-cotton  is  set  on  fire. 

It  would  be  easy  to  explode  gunpowder  with  the  electric  spark, 
but  the  smoke  of  the  explosion  would  make  the  lecture-hall  very  un- 
pleasant for  the  remainder  of  the  lecture.  I  propose,  therefore,  to 


LIGHTNING  AND  THUNDER.  33 

substitute  for  gunpowder  an  explosive  mixture  of  oxygen  and  hydro- 
gen, with  which  I  have  filled  this  little  metal  flask,  commonly  known 
as  Volta's  pistol.  By  a  very  simple  contrivance,  the  electric  spark  is 
discharged  through  the  mixture,  when  I  hold  the  flask  toward  the 
conductor  of  the  machine.  A  cork  is  fitted  tightly  into  the  neck  of 
the  flask,  and  at  the  moment  the  spark  passes  you  hear  a  loud  explo- 
sion, and  you  see  the  cork  driven  violently  up  to  the  ceiling. 

Destruction  of  Life. — The  last  effect  of  lightning  to  which  1 
shall  refer,  and  which,  perhaps,  more  than  any  other,  strikes  us  with 
terror,  is  the  sudden  and  utter  extinction  of  life,  when  the  lightning 
flash  descends  on  man  or  on  beast.  So  swift  is  this  effect,  in  most 
cases,  that  death  is,  in  all  probability,  absolutely  painless,  and  the  vic- 
tim is  dead  before  he  can  feel  that  he  is  struck.  I  cannot  give  you, 
with  any  degree  of  exactness,  the  number  of  people  killed  every  year 
by  lightning,  because  the  record  of  such  deaths  has  been  hitherto  very 
imperfectly  kept,  in  almost  all  countries,  and  is,  beyond  doubt,  very 


GUN-COTTON   SET   ON    FIRE    BY   ELECTRIC   SPARK. 


incomplete.  But  perhaps  you  will  be  surprised  to  learn  that  the  num- 
ber of  deaths  by  lightning  actually  recorded  is,  on  an  average,  in 
England  about  22  every  year,  in  France  80,  in  Prussia  no,  in  Austria 
212,  in  European  Russia  440.' 

So  far  as  can  be  gathered  from  the  existing  sources  of  information, 
it  would  seem  that  the  number  of  persons  killed  by  lightning  is,  on 
the  whole,  about  one  in  three  of  those  who  are  struck.  The  rest  are 
sometimes  only  stunned,  sometimes  more  or  less  burned,  sometimes 
made  deaf  for  a  time,  sometimes  partially  paralyzed.  On  particular 
occasions,  however,  especially  when  the  lightning  falls  on  a  large  as- 
sembly of  people,  the  number  of  persons  struck  down  and  slightly  in- 
jured, in  proportion  to  the  number  killed,  is  very  much  increased. 

An  interesting  case  of  this  kind  is  reported  by  Mr.  Tomlinson. 
"  On  the  twenty-ninth  of  August,  1847,  at  the  parish  church  of  Welton, 

1  See  Anderson,  Lightning  Conductors,  pp.  170-5. 


34  LIGHTNING  AND  THUNDER. 

Lincolnshire,  while  the  congregation  were  engaged  in  singing  the 
hymn  before  the  sermon,  and  the  Rev.  Mr.  Williamson  had  just  as- 
cended the  pulpit,  the  lightning  was  seen  to  enter  the  church  from 
the  belfry,  and  instantly  an  explosion  occurred  in  the  centre  of  the 
edifice.  All  that  could  move  made  for  the  door,  and  Mr.  Williamson 
descended  from  the  pulpit,  endeavoring  to  allay  the  fears  of  the  peo- 
ple. But  attention  was  now  called  to  the  fact  that  several  of  the 
congregation  were  lying  in  different  parts  of  the  church,  apparently 
dead,  some  of  whom  had  their  clothing  on  fire.  Five  women  were 
found  injured,  and  having  their  faces  blackened  and  burned,  and  a 
boy  had  his  clothes  almost  entirely  consumed.  A  respected  old  par- 
ishioner, Mr.  Brownlow,  aged  sixty-eight,  was  discovered  lying  at  the 
bottom  of  his  pew,  immediately  beneath  one  of  the  chandeliers,  quite 
dead.  There  were  no  marks  on  the  body,  but  the  buttons  of  his 


VOLTA'S  PISTOL  ;  EXPLOSION  CAUSED  BY  ELECTRIC  SPARK. 

waistcoat  were  melted,  the  right  leg  of  his  trousers  torn  down,  and  his 
coat  literally  burnt  off.  His  wife  in  the  same  pew  received  no  injury."  ' 
Not  less  striking  is  the  story  told  by  Dr.  Plummer,  surgeon  of  the 
Illinois  Volunteers,  in  the  Medical  and  Surgical  Reporter  of  June  19, 
1865:  "Our  regiment  was  yesterday  the  scene  of  one  of  the  most 
terrible  calamities  which  it  has  been  my  lot  to  witness.  About  two 
o'clock  a  violent  thunderstorm  visited  us.  While  the  old  guard  was 
being  turned  out  to  receive  the  new,  a  blinding  flash  of  lightning  was 
seen,  accompanied  instantly  by  a  terrific  peal  of  thunder.  The  whole 
of  the  old  guard,  together  with  part  of  the  new,  were  thrown  violently 
to  the  earth.  The  shock  was  so  severe  and  sudden  that,  in  most 
cases,  the  rear  rank  men  were  thrown  across  the  front  rank  men.  One 

1  The  Thunderstorm,  pp.  158-9.  See  also  an  account  of  four  persons  who  were  struck  on  the  Mat- 
terhorn,  in  July,  1869,  all  of  whom  were  hurt,  and  none  killed  :  Whymper's  Scrambles  Among  the  Alps, 
PP-  4H,  415- 


LIGHTNING  AND  THUNDER. 


35 


man  was  instantly  killed,  and  thirty-two  men  were  more  or  less  se- 
verely burned  by  the  electric  fluid.  In  some  instances  the  men's 
boots  and  shoes  were  rent  from  their  feet  and  torn  to  pieces,  and, 
strange  as  it  may  appear,  the  men  were  injured  but  little  in  the  feet. 
In  all  cases  the  burns  appear  as  if  they  had  been  caused  by  scalding- 
hot  water,  in  many  instances  the  skin  being  shriveled  and  torn  off. 
The  men  all  seem  to  be  doing  well,  and  a  part  of  them  will  be  able  to 
resume  their  duties  in  a  few  days." 

The  Return  Shock. — It  sometimes  happens  that  people  are 
struck  down  and  even  killed  at  the  moment  a  discharge  of  lightning 
takes  place  between  a  cloud  and  the  earth,  though  they  are  very  far 
from  the  point  where  the  flash  is  actually  seen  to  pass  ;  while  others, 
who  are  situated  between  them  and  the  lightning,  suffer  very  little,  or 
perhaps  not  at  all.  This  curious  phenomenon  was  first  carefully  in- 
vestigated by  Lord  Mahon  in  the  year  1779,  and  was  called  by  him 


THE    RETURN    SHOCK    ILLUSTRATED. 


the  "return  shock."     His  theory,  which  is  now  commonly  accepted, 
may  be  easily  understood  with  the  aid  of  the  sketch  before  you. 

Let  us  suppose  ABC  to  represent  the  outline  of  a  thundercloud 
which  dips  down  toward  the  earth  at  A  and  at  c.  The  electricity  of 
the  cloud  develops  by  inductive  action  a  charge  of  the  opposite  kind 
in  the  earth  beneath  it.  But  the  inductive  action  is  most  powerful  at 
E  and  F,  where  the  cloud  comes  nearest  to  the  earth.  Hence,  bodies 
situated  near  these  points  may  be  very  highly  electrified  as  compared 
with  bodies  at  a  point  between  them,  such  as  D.  Now,  when  a  flash 
of  lightning  passes  at  E,  the  under  part  of  the  cloud  is  at  once  relieved 
of  its  electricity,  its  inductive  action  ceases,  and,  therefore,  a  person 
situated  at  F  suddenly  ceases  to  be  electrified.  This  sudden  change 
from  a  highly  electrified  to  a  neutral  state  involves  a  shock  to  his 
system  which  may  be  severe  enough  to  stun  or  even  to  kill  him. 


36  LIGHTNING  AND  THUNDER. 

Meanwhile,  people  at  r>,  having  been  also  electrified  to  some  extent 
by  the  influence  of  the  thundercloud,  must  in  like  manner  undergo  a 
change  in  their  electrical  condition  when  the  flash  of  lightning  passes, 
but  this  change  will  be  less  violent  because  they  were  less  highly 
electrified. 

Many  experiments  have  been  devised  to  illustrate  this  theory  of 
Lord  Mahon.  But  the  best  illustration  I  know  is  furnished  by  this 
electric  machine  of  Carre's.  If  you  stand  near  one  end  of  the  large 
conductor  when  the  machine  is  in  action  and  sparks  are  taken  from 
the  other  end,  you  will  feel  a  distinct  electric  shock  every  time  a  spark 
passes.  The  large  conductor  here  takes  the  place  of  the  cloud,  the 
spark  that  passes  at  one  end  represents  the  flash  of  lightning,  and  the 
observer  at  the  other  end  gets  the  return  shock,  though  he  is  at  a 
considerable  distance  from  the  point  where  the  flash  is  seen. 

An  experiment  of  this  kind,  of  course,  cannot  be  made  sensible  to 
a  large  audience  like  the  present.  But  I  can  give  you  a  good  idea  of 
the  effect  by  means  of  this  tuft  of  colored  papers.  While  the  machine 
is  in  action  I  hold  the  tuft  of  papers  near  that  end  of  the  conductor 
which  is  farthest  from  the  point  where  the  discharge  takes  place. 
You  see  the  paper  ribbons  are  electrified  by  induction,  and,  in  virtue, 
of  mutual  repulsion,  stand  out  from  one  another  "like  quills  upon  the 
fretful  porcupine."  But,  when  a  spark  passes,  the  inductive  action 
ceases,  the  paper  ribbons  cease  to  be  electrified,  and  the  whole  tuft 
suddenly  collapses  into  its  normal  state. 

While  fully  accepting  Lord  Mahon's  theory  of  the  return  shock 
as  perfectly  good  so  far  as  it  goes,  I  would  venture  to  point  out  an- 
other influence  which  must  often  contribute  largely  to  produce  the 
effect  in  question,  and  which  is  not  dependent  on  the  form  of  the 
cloud.  It  may  easily  happen,  from  the  nature  of  the  surface  in  the 
district  affected  by  a  thundercloud,  that  the  point  of  most  intense 
electrification — say  E  in  the  figure — is  in  good  electrical  communica- 
tion with  a  distant  point,  such  as  F,  while  it  is  very  imperfectly  con- 
nected with  a  much  nearer  point,  D.  In  such  a  case  it  is  evident  that 
bodies  at  F  will  share  largely  in  the  highly-electrified  condition  of  E, 
and  also  share  largely  in  the  sudden  change  of  that  condition  the 
moment  the  flash  of  lightning  passes  ;  whereas  bodies  at  D  will  be  less 
highly  electrified  before  the  discharge,  and  less  violently  disturbed 
when  the  discharge  takes  place. 

This  principle  may  be  illustrated  by  a  very  simple  experiment. 
Here  is  a  brass  chain  about  twenty  feet  long.  One  end  of  it  I  hand 
to  any  one  among  the  audience  who  will  kindly  take  hold  of  it ;  the 
other  end  I  hold  in  my  hand.  I  now  stand  near  the  conductor  of  the 
machine  ;  and  will  ask  some  one  to  stand  about  ten  feet  away  from 
me,  near  the  middle  of  the  chain,  but  without  touching  it.  Now  ob- 
serve what  happens  when  the  machine  is  worked  and  I  take  a  spark 


LIGHTNING  AND  THUNDER.  37 

from  the  conductor  :  My  friend  at  the  far  end  of  the  chain,  twenty 
feet  away,  gets  a  shock  nearly  as  severe  as.  the  one  I  get  myself,  be- 
cause he  is  in  good  electrical  communication  with  the  point  where  the 
discharge  takes  place.  But  my  more  fortunate  friend,  who  is  ten  feet 
nearer  to  the  flash,  is  hardly  sensible  of  any  effect,  because  he  is  con- 
nected with  me  only  through  the  floor  of  the  hall,  which  is,  compara- 
tively speaking,  a  bad  conductor  of  electricity. 

Summary. — Let  me  now  briefly  sum  up  the  chief  destructive 
effects  of  lightning.  First,  with  regard  to  good  conductors :  though 
it  passes  harmlessly  through  them  if  they  be  large  enough  to  afford  it 
an  easy  passage,  it  melts  and  converts  them  into  vapor  if  they  be  of 
such  small  dimensions  as  to  offer  considerable  resistance.  Secondly, 
lightning  acts  with  great  mechanical  force  on  bad  conductors  ;  it  is 
capable  of  tearing  asunder  large  masses  of  masonry,  and  of  project- 
ing the  fragments  to  a  considerable  distance.  Thirdly,  it  sets  fire  to 
combustible  materials.  And  lastly,  it  causes  the  instantaneous  death 
of  men  and  animals. 

Franklin's  Lightning  Rods. — The  object  of  lightning  con- 
ductors is  to  protect  life  and  property  from  these  destructive  effects. 
Their  use  was  first  suggested  by  Franklin,  in  1749,  even  before  his 
famous  experiment  with  the  kite  ;  and  immediately  after  that  experi- 
ment, in  1752,  he  set  up,  on  his  own  house,  in  Philadelphia,  the  first 
lightning  conductor  ever  made.  He  even  devised  an  ingenious  con- 
trivance, by  means  of  which  he  received  notice  when  a  thundercloud 
was  approaching.  The  contrivance  consisted  of  a  peal  of  bells,  which 
he  hung  on  his  lightning  conductor,  and  which  were  set  ringing  when- 
ever the  lightning  conductor  became  charged  with  electricity. 

Franklin's  lightning  rods  were  soon  adopted  in  America  ;  and  he 
himself  contributed  very  much  to  their  popularity  by  the  simple  and 
lucid  instructions  he  issued  every  year,  for  the  benefit  of  his  country- 
men, in  the  annual  publication  known  as  "Poor  Richard's  Almanac." 
It  is  very  interesting  at  this  distance  of  time  to  read  the  homely  prac- 
tical rules  laid  down  by  this  great  philosopher  and  statesman  ;  and, 
though  some  modifications  have  been  suggested  by  the  experience  of 
a  hundred  and  thirty  years,  especially  as  regards  the  dimensions  of 
the  lightning  conductor,  it  is  surprising  to  find  how  accurately  the 
general  principles  of  its  construction,  and  of  its  action,  are  here  set 
forth. 

"It  has  pleased  God,"  he  says,  "in  His  goodness  to  mankind,  at 
length  to  discover  to  them  the  means  of  securing  their  habitations 
and  other  buildings  from  mischief  by  thunder  and  lightning.  The 
method  is  this  :  Provide  a  small  iron  rod,  which  may  be  made  of  the 
rod-iron  used  by  nailors,  but  of  such  a  length  that  one  end  being 
three  or  four  feet  in  the  moist  ground,  the  other  may  be  six  or  eight 
feet  above  the  highest  part  of  the  building.  To  the  upper  end  of  the 


38  LIGHTNING  AND  THUNDER. 

rod  fasten  about  a  foot  of  brass  wire,  the  size  of  a  common  knitting 
needle,  sharpened  to  a  fine  point  ;  the  rod  may  be  secured  on  the 
house  by  a  few  small  staples.  If  the  house  or  barn  be  long,  there 
may  be  a  rod  and  point  at  each  end,  and  a  middling  wire  along  the 
ridge  from  one  to  the  other.  A  house  thus  furnished  will  not  be  dam- 
aged by  lightning,  it  being  attracted  by  the  points  and  passing  through 
the  metal  into  the  ground,  without  hurting  anything.  Vessels  also 
having  a  sharp-pointed  rod  fixed  on  the  top  of  their  masts,  with  a 
wire  from  the  foot  of  the  rod  reaching  down  round  one  of  the  shrouds 
to  the  water,  will  not  be  hurt  by  lightning." 

Introduction  of  Lightning  Rods  into  England. — The  pro- 
gress of  lightning  conductors  was  more  slow  in  England  and  on  the 
Continent  of  Europe,  owing  to  a  fear,  not  unnatural,  that  they  might, 
in  some  cases,  draw  down  the  lightning  where  it  would  not  otherwise 
have  fallen.  People  preferred  to  take  their  chance  of  escaping  as 
they  had  escaped  before,  rather  than  invite,  as  it  were,  the  lightning 
to  descend  on  their  houses,  in  the  hope  that  an  iron  rod  would  convey 
it  harmless  to  the  earth.  But  the  immense  amount  of  damage  done  every 
year  by  lightning,  soon  led  practical  men  to  entertain  a  proposal  which 
offered  complete  immunity  from  all  danger  on  such  easy  terms  ;  and 
when  it  was  found  that  buildings  protected  by  lightning  conductors 
were,  over  and  over  again,  struck  by  lightning  without  suffering  any 
harm,  a  general  conviction  of  their  utility  was  gradually  established 
in  the  public  mind. 

The  first  public  building  protected  by  a  lightning  rod  in  England 
was  St.  Paul's  Cathedral,  in  London.  On  the  eighteenth  of  June, 
1764,  the  beautiful  steeple  of  Saint  Bride's  Church,  in  the  city,  was 
struck  by  lightning  and  reduced  to  ruin.  This  incident  awakened  the 
attention  of  the  dean  and  chapter  of  St.  Paul's  to  the  danger  of  a 
similar  calamity,  which  seemed,  as  it  were,  impending  over  their  own 
church.  After  long  deliberation,  they  referred  the  matter  to  the  Royal 
Society,  asking  for  advice  and  instruction.  A  committee  of  scientific 
men  was  appointed  by  the  Royal  Society  to  consider  the  question. 
Benjamin  Franklin  himself,  who  happened  to  be  in  London  at  the 
time,  as  the  representative  of  the  American  States  in  their  dispute 
with  England,  was  nominated  a  member  of  the  committee.  And  the 
result  of  its  deliberation  was  that,  in  the  year  1769,  a  number  of 
lightning  conductors  were  erected  on  St.  Paul's  Cathedral. 

It  was  on  this  occasion  that  arose  the  celebrated  controversy  about 
the  respective  merits  of  points  and  balls.  Franklin  had  recommended 
a  pointed  conductor  ;  but  some  members  of  the  committee  were  of 
opinion  that  the  conductor  should  end  in  a  ball  and  not  in  a  point. 
The  decision  of  the  committee  was  in  favor  of  Franklin's  opinion,  and 
pointed  conductors  were  accordingly  adopted  for  St.  Paul's  Cathedral. 
But  the  controversy  did  not  end  here.  The  time  was  one  of  great 


LIGHTNING  AND  THUNDER.  39 

political  excitement,  and  party  spirit  infused  itself  even  into  the  peace- 
ful discussions  of  science.  The  weight  of  scientific  opinion  was  on 
the  side  of  Franklin  ;  but  it  was  hinted,  on  the  other  side,  that  the 
pointed  conductors  were  tainted  with  republicanism,  and  pregnant 
with  danger  to  the  empire.  As  a  rule,  the  whigs  were  strongly  in 
favor  of  points  ;  while  the  Tories  were  enthusiastic  in  their  support  of 
balls. 

For  a  time  the  Tories  seemed  to  prevail.  The  king  was  on  their 
side.  Experiments  on  a  grand  scale  were  conducted  in  his  presence, 
at  the  Pantheon,  a  large  building  in  Oxford  street  ;  he  was  assured 
that  these  experiments  proved  the  great  superiority  of  balls  over 
points  ;  and  to  give  practical  effect  to  his  convictions,  his  majesty 
directed  that  a  large  cannon  ball  should  be  fixed  on  the  end  of  the 
lightning  conductor  attached  to  the  royal  palace  at  Kew.  But  the 
committee  of  the  Royal  Society  remained  unconvinced.  In  course  of 
time  the  heat  of  party  spirit  abated;  experience  as  well  as  reason 
was  found  to  be  in  favor  of  Franklin's  views  ;  and  the  battle  of  the 
balls  and  points  has  long  since  passed  into  the  domain  of  history.1 

Functions  of  a  Lightning  Conductor. — A  lightning  conductor 
fulfills  two  functions.  First,  it  favors  a  silent  and  gradual  discharge  of 
electricity  between  the  cloud  and  the  earth,  and  thus  tends  to  prevent 
that  accumulation  which  must  of  necessity  take  place  before  a  flash 
of  lightning  will  pass.  Secondly,  if  a  flash  of  lightning  come,  the 
lightning  conductor  offers  it  a  safe  channel  through  which  it  may  pass 
harmless  to  the  earth. 

These  two  functions  of  a  lightning  conductor  may  be  easily  illus- 
trated by  experiment.  When  our  machine  is  in  action,  if  I  present 
my  closed  hand  to  the  large  brass  conductor,  a  spark  passes  between 
them,  and  I  feel,  at  the  same  moment,  a  slight  electric  shock.  Here 
the  conductor  of  the  machine,  as  usual,  holds  the  place  of  the  electri- 
fied cloud  ;  my  closed  hand  represents,  as  it  were,  a  lofty  building 
that  stands  out  prominently  on  the  surface  of  the  earth  ;  the  spark  is 
the  flash  of  lightning,  and  the  electric  shock  just  suggests  the  de- 
structive power  of  the  sudden  disruptive  discharge. 

Now  let  me  protect  this  building  by  a  lightning  conductor.  For 
this  purpose,  I  take  in  my  hand  a  brass  rod,  which  I  connect  with  the 
earth  by  a  brass  chain.  In  the  first  instance,  I  will  have  a  metal  ball 
on  the  end  of  my  lightning  conductor.  You  see  the  effect  ;  sparks 
pass  rapidly,  but  I  feel  no  shock.  I  can  increase  the  strength  of  the 
discharge  by  hanging  this  condensing  jar  on  the  conductor  of  the  ma- 
chine. Sparks  pass  now,  much  more  brilliant  and  powerful  than  be- 
fore, but  still  I  get  no  shock.  It  is  evident,  therefore,  that  my  light- 
ning rod  does  not  prevent  the  flash  from  passing,  but  it  conveys  it 
harmless  to  the  ground. 

1  See  Philosophical  Transactions  of  the  Royal  Society,  1773,  p.  42,  and  1778,  part  i.,  p.  232  ;  Ander- 
son's Lightning  Conductors,  pp.  40-2  ;  Lightning  Rod  Conference,  pp.  76-9. 


40  LIGHTNING  AND  THUNDER. 


. 


I  next  take  a  rod  which  is  sharply  pointed,  and  connecting  it  as 
fore  with  the  earth  by  a  brass  chain,  I  present  the  sharp  point  to  the 
conductor  of  the  machine.  Observe  how  different  is  the  result  ;  there 
is  no  disruptive  discharge  :  no  spark  passes  ;  no  shock  is  felt.  Elec- 
tricity still  continues  to  be  generated  in  the  machine,  and  electricity 
is  generated,  by  induction,  in  the  brass  rod,  and  in  my  body.  But 
these  two  opposite  electricities  discharge  themselves  silently,  by  means 
of  this  pointed  rod,  and  no  sensible  effect  of  any  kind  is  exhibited. 

These  experiments  are  very  simple,  but  they  really  put  before  us, 
in  the  clearest  possible  way,  the  whole  theory  of  lightning  conductors. 
In  particular,  they  give  us  ocular  demonstration  that  an  efficient  light- 
ning rod  not  only  makes  the  lightning  harmless  when  it  comes,  but 
tends  very  much  to  prevent  its  coming.  A  remarkable  example,  on  a 
large  scale,  of  this  important  property,  is  furnished  by  the  town  of 
Pietermaritzburg,  the  capital  of  the  colony  of  Natal,  in  South  Africa. 
This  town  is  subject  to  the  frequent  visitation  of  thunderstorms,  at 
certain  seasons  of  the  year,  and  much  damage  was  formerly  done  by 
lightning,  but  since  the  erection  of  lightning  conductors  on  the  prin- 
cipal buildings,  the  lightning  has  never  fallen  within  the  town. 
Thunderclouds  come  as  before,  but  they  pass  silently  over  the  city, 
and  only  begin  to  emit  their  lightning  flashes  when  they  reach  the 
open  country,  and  have  passed  beyond  the  range  of  the  lightning 
conductors.1 

But  it  will  often  happen,  even  in  the  case  of  a  pointed  conductor, 
that  the  accumulation  of  electricity  goes  on  so  fast  that  the  silent  dis- 
charge is  insufficient  to  keep  it  in  check.  A  disruptive  discharge  will 
then  take  place,  from  time  to  time,  and  a  flash  of  lightning  will  pass. 
Under  these  circumstances,  the  lightning  conductor  is  called  upon  to 
fulfill  its  second  function,  and  to  convey  the  lightning  harmless  to  the 
earth. 

Conditions  of  a  Lightning  Conductor. — From  the  considera- 
tion of  the  functions  which  it  has  to  fulfill,  we  may  now  infer  what  are 
the  conditions  necessary  for  an  efficient  lightning  conductor.  The 
first  condition  is  that  the  end  of  the  conductor,  projecting  into  the  air, 
should  have,  at  least,  one  sharp  point.  Our  experiments  have  shown 
us  that  a  pointed  conductor  tends,  in  a  manner,  to  suppress  the  flash 
of  lightning  altogether  ;  whereas  a  blunt  conductor,  or  one  ending  in 
a  ball,  tends  only  to  make  it  harmless  when  it  comes.  It  is  evident, 
therefore,  that  the  pointed  conductor  offers  the  greater  security. 

But  a  fine  point  is  very  liable  to  be  melted  when  the  lightning  falls 
upon  it,  and  thus  to  be  rendered  less  efficient  for  future  service.  To 
meet  this  danger,  it  has  recently  been  suggested,  by  the  Lightning 
Rod  Conference,  that  the  extreme  end  of  the  conductor  should  be  a 
blunt  point,  destined  to  receive  the  full  force  of  the  lightning  flash, 

1  See  A  Lecture  on  Thunderstorms,  by  Professor  Tait  of  Edinburgh,  published  in  Nature,  vol. 
xxii.,  p.  365. 


LIGHTNING  AND  THUNDER.  41 

when  it  comes  ;  and  that,  a  little  lower  down,  a  number  of  very  fine 
points  should  be  provided,  with  a  view  to  favor  the  silent  discharge. 
This  suggestion,  which  appears  admirably  fitted  to  provide  for  the 
twofold  function  of  a  lightning  conductor,  deserves  to  be  recorded  in 
the  exact  terms  of  the  official  report. 

"  It  seems  best  to  separate  the  double  functions  of  the  point,  pro- 
longing the  upper  terminal  to  the  very  summit,  and  merely  beveling 
it  off,  so  that,  if  a  disruptive  discharge  does  take  place,  the  full  con- 
conducting  power  of  the  rod  may  be  ready  to  receive  it.  At  the  same 
time,  having  regard  to  the  importance  of  silent  discharge  from  sharp 
points,  we  suggest  that,  at  one  foot  below  the  extreme  top  of  the 
upper  terminal,  there  be  firmly  attached,  by  screws  and  solder,  a  cop- 
per ring  bearing  three  or  four  copper  needles,  each  six  inches  long, 
and  tapering  from  a  quarter  of  an  inch  diameter  to  as  fine  a  point  as 
can  be  made  ;  and  with  the  object  of  rendering  the  sharpness  as  per- 
manent as  possible,  we  advise  that  they  be  platinized,  gilded,  or 
nickel  plated."1 

The  second  condition  of  a  lightning  conductor  is,  that  it  should  be 
made  of  such  material,  and  of  such  dimensions,  as  to  offer  an  easy 
passage  to  the  greatest  flash  of  lightning  likely  to  fall  on  it ;  other- 
wise it  might  be  melted  by  the  discharge,  and  the  lightning,  seeking 
for  itself  another  path,  might  force  its  way  through  bad  conductors, 
which  it  would  partly  rend  asunder,  and  partly  consume  by  fire.  Cop- 
per is  now  generally  regarded  as  the  best  material  for  lightning  con- 
ductors, and  it  is  almost  universally  employed  in  these  countries.  If 
it  is  used  in  the  form  of  a  rope,  it  should  not  be  less  than  half  an  inch 
in  diameter  ;  if  a  band  of  copper  is  preferred — and  it  is  often  found 
more  convenient  by  builders — it  should  be  about  an  inch  and  a  half 
broad  and  an  eighth  of  an  inch  thick.  In  France  it  has  been  hitherto 
more  usual  to  employ  iron  rods  for  lightning  conductors,  but  since 
iron  is  much  inferior  to  copper  in  its  conducting  power,  the  iron  rod 
must  be  of  much  larger  dimensions  ;  it  should  be  at  least  one  inch  in 
diameter.2 

The  third  condition  is  that  the  lightning  conductor  should  be 
continuous  throughout  its  whole  length,  and  should  be  placed  in  good 
electrical  contact  with  the  earth.  This  is  a  condition  of  the  first  im- 
portance, and  experience  has  shown  that  it  is  the  one  most  likely  of 
all  to  be  neglected.  In  a -large  town  the  best  earth  connection  is  fur- 
nished by  the  system  of  water-mains  and  gas-mains,  each  of  which 
constitutes  a  great  network  of  conductors  everywhere  in  contact  with 

1  Report  of  the  Lightning  Rod  Conference,  p.  4. 

3  The  dimensions  here  set  forth  are  greater  in  some  respects  than  those  "  recommended  as  a  mini- 
mum" in  the  report  of  the  Lightning  Rod  Conference,  page  6.  But  it  will  be  oboeryed  by  those  who 
consult  the  report  that  the  minimum  recommended  is  just  the  size  which,  in  the  preceding  paragraph  of 
the  report,  is  said  to  have  been  actually  melted  by  a  flash  of  lightning  ;  and,  therefore,  it  seems  not  to 
be  a  very  safe  minimum.  It  wiii  be  also  seen  that  there  is  some  confusion  in  the  figures  given,  and  that 
they  contradict  one  another.  For  the  dimensions  of  iron  rods,  see  the  instructions  adopted  by  the 
Academy  of  Science,  Paris,  May  ao,  1875  ;  Lightning  Rod  Conference,  pp.  67-8. 


42  LIGHTNING  AND  THUNDER. 

the  earth.  Two  points,  however,  must  be  carefully  attended  to — first, 
that  the  electrical  contact  between  the  lightning  conductor  and  the 
metal  pipe  should  be  absolutely  perfect ;  and,  secondly,  that  the  pipe 
selected  should  be  of  such  large  dimensions  as  to  allow  the  lightning 
an  easy  passage  through  it  to  the  principal  main. 

If  no  such  system  of  water-pipes  or  gas-pipes  is  at  hand,  then  the 
lightning  rod  should  be  connected  with  moist  earth  by  means  of  a 
bed  of  charcoal  or  a  metal  plate  not  less  than  three  feet  square.  This 
metal  plate  should  be  always  of  the  same  material  as  the  conductor, 
otherwise  a  galvanic  action  would  be  set  up  between  the  two  metals, 
which  in  course  of  time  might  seriously  damage  the  contact.  Dry 
earth,  sand,  rock,  and  shingle  are  bad  conductors  ;  and,  if  such  mate- 
rials exist  near  the  surface  of  the  earth,  the  lightning  rod  must  pass 
through  them  and  be  carried  down  until  it  reaches  water  or  perma- 
nently damp  earth. 

Mischief  Done  by  Bad  Conductors. — If  the  earth  contact  is 
bad,  a  lightning  conductor  does  more  harm  than  good.  It  invites  the 
lightning  down  upon  the  building  without  providing  for  it,  at  the  same 
time,  a  free  passage  to  earth.  The  consequence  is  that  the  lightning 
forces  a  way  for  itself,  violently  bursting  asunder  whatever  opposes 
its  progress,  and  setting  fire  to  whatever  is  combustible. 

I  will  give  you  some  recent  and  striking  examples.  In  the  month 
of  May,  1879,  the  church  of  Laughton-en-le-Morthen,  in  England, 
though  provided  with  a  conductor,  was  struck  by  lightning  and  sus- 
tained considerable  damage.  On  examination  it  was  found  that  the 
lightning  followed  the  conductor  down  along  the  spire  as  far  as  the 
roof ;  then,  changing  its  course,  it  forced  its  way  through  a  buttress 
of.  massive  masonwork,  dislodging  about  two  cartloads  of  stones,  and 
leaped  over  to  the  leads  of  the  roof,  about  six  feet  distant.  It  now 
followed  the  leads  until  it  came  to  the  cast-iron  down-pipes  intended 
to  discharge  the  rain-water,  and  through  these  it  descended  to  the 
earth.  When  the  earth  contact  of  the  lightning  conductor  was  ex- 
amined, it  was  found  exceedingly  deficient.  The  rod  was  simply  bent 
underground,  and  buried  in  dry  loose  rubbish  at  a  depth  not  exceed- 
ing eighteen  inches.  This  is  a  very  instructive  example.  The  light- 
ning had  a  choice  of  two  paths — one  by  the  conductor  prepared  for 
it,  the  other  by  the  leads  of  the  roof  and  the  down-pipes — and,  by  a 
kind  of  instinct  which,  however  we  may  explain,  we  must  always  con- 
template with  wonder,  it  chose  the  path  of  least  resistance,  though  in 
doing  so  it  had  to  burst  its  way  at  the  outset  through  a  massive  wall 
of  solid  masonry.1 

On  the  5th  of  June,  in  the  same  year,  a  flash  of  lightning  struck  the 
house  of  Mr.  Osbaldiston,  near  Sheffield,  and,  notwithstanding  the 
supposed  protection  of  a  lightning  conductor,  it  did  damage  to  the 

1  See  letter  of  Mr.  R.  S.  Newall,  F.  R.  SM  in  the  Times •,  May  30,  1879. 


LIGHTNING  AND  THUNDER.  43 

amount  of  about  five  hundred  pounds.  The  lightning  here  followed 
the  conductor  to  a  point  about  nine  feet  from  the  ground,  then  passed 
through  a  thick  wall  to  a  gas-pipe  at  the  back  of  the  drawing-room  mir- 
ror. It  melted  the  gas-pipe,  set  fire  to  the  gas,  smashed  the  mirror  to 
atoms,  broke  the  Sevres  vases  on  the  chimney-piece,  and  dashed  the 
furniture  about.  In  this  case,  as  in  the  former,  it  was  found  that  the 
earth  contact  was  bad;  and,  in  addition,  the  conductor  itself  was  of  too 
small  dimensions.  Hence,  the  electric  discharge  found  an  easier  path 
to  earth  through  the  gas-pipes,  though  to  reach  them  it  had  to  force 
for  itself  a  passage  through  a  resisting  mass  of  non-conductors.1 

Again  in  the  same  year,  on  the  28th  of  May,  the  house  of  Mr. 
Tomes,  of  Caterham,  was  struck  by  lightning,  and  some  slight  dam- 
age was  done.  After  a  careful  examination  it  was  found  that  the 
greater  part  of  the  discharge  left  the  lightning  conductor  with  which 
the  house  was  provided,  and  passed  over  the  slope  of  the  roof  to  an 
attic  room,  into  which  it  forced  its  way  through  a  brick  wall,  and 
reached  a  small  iron  cistern.  This  cistern  was  connected  by  an  iron 
pipe  of  considerable  dimensions  with  two  pumps  in  the  basement 
story  ;  and  through  them  the  lightning  found  an  easy  passage  to  the 
earth,  and  did  but  little  harm  on  its  way.  When  the  earth  contact  of 
the  lightning  conductor  was  examined,  it  was  discovered  that  the  end 
of  the  rod  was  simply  stuck  into  a  dry  chalky  soil  to  a  depth  of  about 
twelve  inches.  Thus  in  this  case,  as  in  the  two  former,  it  was  made 
quite  clear  that  the  lightning  conductor  failed  to  fulfill  its  functions 
because  the  earth  contact  was  bad.2 

Cases  are  not  uncommon  in  which  builders  provide  underground  a 
carefully  constructed  reservoir  of  water,  into  which  the  lower  end  of 
the  lightning  rod  is  introduced.  The  idea  seems  to  prevail  that  a 
reservoir  of  water  constitutes  a  good  earth  contact  ;  and  this  is  quite 
true  of  a  natural  reservoir,  such  as  a  lake,  where  the  water  is  in  con- 
tact with  moist  earth  over  a  considerable  area.  But  an  artificial  res- 
ervoir may  have  quite  an  opposite  character,  and  practically  insulate 
the  lightning  conductor  from  the  earth.  One  which  came  under  my 
notice  lately,  in  the  neighborhood  of  this  city,  consists  of  a  large 
earthenware  pipe  set  on  end  in  a  bed  of  cement,  and  kept  half  full  of 
water.  Now,  the  earthenware  pipe  is  a  good  insulator,  and  so  is  the 
bed  of  cement  in  which  it  rests  ;  and  the  whole  arrangement  is  identi- 
cal, in  all  essential  features,  with  the  apparatus  of  Professor  Richman, 
in  which  he  introduced  his  lightning  rod  into  a  glass  bottle,  and  by 
which  he  lost  his  life  a  hundred  and  thirty  years  ago. 

A  conductor  mounted  in  this  manner  will,  probably  enough,  draw 
down  lightning  from  the  clouds  ;  but  it  is  more  likely  to  discharge  it, 
with  destructive  effect,  into  the  building  it  is  intended  to  guard,  than 
to  transmit  it  harmlessly  to  the  earth.  An  example  is  at  hand  in  the 

1  See  Nature,  June  12,  1879,  vol.  xx.,  p.  146. 

8  See  letter  of  Mr.  Tomes  in  Nature,  vol.  xx.,  p.  145  ;  also  Lightning  Rod  Conference,  pp.  210-15. 


44  LIGHTNING  AND  THUNDER. 

case  of  Christ  Church,  in  the  town  of  Clevedon,  in  Somersetshire. 
This  church  was  provided  with  a  very  efficient  system  of  lightning 
conductors,  five  in  number,  corresponding  to  the  four  pinnacles  and  the 
flagstaff,  on  the  summit  of  the  principal  tower.  The  five  conductors 
consisted  of  good  copper-wire  rope  ;  all  were  united  together  inside 
the  tower,  through  which  they  were  carried  down  to  earth,  and  there 
ended  in  an  earthenware  drain.  This  kind  of  earth  contact  might  be 
pretty  good  as  long  as  water  was  flowing  in  the  drain  ;  but  whenever 
the  drain  was  dry  the  conductor  was  practically  insulated  from  the 
earth.  On  the  fifteenth  of  March,  1876,  the  church  was  struck  by 
lightning,  which  for  some  distance  followed  the  line  of  the  conductor; 
then  finding  its  passage  barred  by  the  earthenware  drain,  which  was 
dry  at  the  time,  it  burst  through  the  walls  of  the  church,  displacing 
several  hundredweight  of  stone,  and  making  its  way  to  earth  through 
the  gas-pipe.1 

Another  very  instructive  example  is  furnished  by  the  lightning  con- 
ductor attached  to  the  lighthouse  of  Berehaven,  on  the  south-west 
coast  of  Ireland.  It  consists  of  a  half-inch  copper-wire  rope,  which 
is  carried  down  the  face  of  the  tower  "  until  it  reaches  the  rock  at 
it  base,  where  it  terminates  in  a  small  hole,  three  inches  by  three  inches, 
jumped  out  of  the  rock,  about  six  inches  under  the  surface"  Here,  again, 
we  have  a  good  imitation  of  Professor  Richman's  experiment,  with 
only  this  difference,  that  a  small  hole  in  the  rock  is  substituted  for  a 
glass  bottle.  A  lightning  conductor  of  this  kind  fulfills  two  functions: 
it  increases  the  chance  of  the  lightning  coming  down  on  the  building, 
and  it  makes  it  positively  certain  that,  having  come,  it  cannot  get  to 
earth  without  doing  mischief. 

The  lightning  did  come  down  on  the  Berehaven  Lighthouse,  about 
five  years  ago.  As  might  have  been  expected,  it  made  no  use  of  the 
lightning  conductor  in  finding  a  path  to  earth,  but  forced  its  way 
through  the  building,  dealing  destruction  around  as  it  descended  from 
stage  to  stage.  The  Board  of  Irish  Lights  furnished  a  detailed  report 
of  this  accident  to  the  Lightning  Rod  Conference,  in  March,  1880,  from 
which  the  above  particulars  have  been  derived.* 

Precaution  Against  Rival  Conductors.— But  it  is  not  enough 
to  provide  a  good  lightning  conductor,  which  is  itself  able  to  convey 
the  electric  discharge  harmless  to  the  earth  ;  we  must  take  care  that 
there  are  no  rival  conductors  near  at  hand  in  the  building,  to  draw  off 
the  lightning  from  the  path  prepared  for  it,  and  conduct  it  by  another 
route  in  which  its  course  might  be  marked  with  destruction.  This 
precaution  is  of  especial  importance  at  the  present  day,  owing  to  the 
great  extent  to  which  metal,  of  various  kinds,  is  employed  in  the  con- 
struction and  fittings  of  modern  buildings.  I  will  take  a  typical  case 
which  will  bring  home  this  point  clearly  to  your  minds. 

1  See  Anderson,  Lightning  Conductors,  pp.  208-10. 

8  See  Lightning  Rod  Conference,  pp.  208-10  ;  see  also  the  note  at  the  end  of  this  Lecture,  p.  §3, 


LIGHTNING  AND  THUNDER.  45 

A  great  part  of  the  roof  of  many  large  buildings  is  covered  with 
lead.  The  lead,  at  one  or  more  points  may  come  near  the  gutters  in- 
tended to  collect  the  rain  water  ;  the  gutters  are  in  connection  with 
the  cast-iron  down-pipes  into  which  the  water  flows,  and  these  down- 
pipes  often  pass  into  the  earth,  which,  under  the  circumstances,  is 
generally  moist,  and,  therefore,  in  good  electrical  contact  with  the 
metal  pipes.  Here,  then,  is  an  irregular  line  of  conductors,  which, 
though  it  has  gaps  here  and  there,  may,  under  certain  conditions, 
offer  to  the  lightning  discharge  a  path  not  less  free  than  the  lightning 
conductor  itself.  What  is  the  consequence  ?  The  flash  of  lightning,  or 
a  part  of  it,  will  quit  the  lightning  rod,  and  make  its  way  to  earth 
through  the  broken  series  of  conductors,  doing,  perhaps,  serious  mis- 
chief, as  it  leaps  across,  or  bursts  asunder,  the  non-conducting  links 
in  the  chain. 

Another  illustration  may  be  taken  from  the  gas  and  water-pipes, 
with  which  almost  all  buildings  in  great  cities  are  now  provided,  and 
which  constitute  a  network  of  conductors,  spreading  out  over  the 
walls  and  ceilings,  and  stretching  down  into  the  earth,  with  which  they 
have  the  best  possible  electrical  contact.  Now,  it  often  happens  that 
a  lightning  conductor,  at  some  point  in  its  course,  comes  within  a  short 
distance  of  this  network  of  pipes.  In  such  a  case,  a  portion  of  the  elec- 
trical discharge  is  apt  to  leave  the  lightning  conductor,  force  .its  way 
destructively  through  masses  of  masonry,  enter  the  network  of  pipes, 
melt  the  leaden  gas-pipe,  ignite  the  gas,  and  set  the  building  on  fire. 

These  are  not  merely  the  speculations  of  philosophers.  All  the 
various  incidents  I  have  just  described  have  occurred,  over  and  over 
again,  during  the  last  few  years.  You  will  remember,  in  some  of  the 
examples  I  have  already  set  before  you,  when  the  electric  discharge 
failed  to  find  a  sufficient  path  to  earth  through  the  lightning  rod,  it 
followed  some  such  broken  series  of  chance  conductors  as  we  are  now 
considering.  But  this  broken  series  of  conductors  seems  to  bring 
with  it  a  special  danger  of  its  own,  even  when  the  lightning  conductor 
is  otherwise  in  efficient  working  order.  I  will  give  you  just  one  case 
in  point. 

On  the  fifth  of  June,  1879,  the  Church  of  Saint  Marie,  Rugby,  was 
struck  by  lightning  and  set  on  fire,  and  narrowly  escaped  being  burned 
to  the  ground.  A  number  of  workmen  were  engaged  on  that  day  in 
repairing  the  spire  of  the  church.  About  three  o'clock  they  saw  a 
dense  black  cloud  approaching,  and  they  came  down  to  take  shelter 
within  the  building.  In  a  few  minutes  they  heard  a  terrific  crash  just 
overhead  ;  at  the  same  moment  the  gas  was  lighted  under  the  organ 
loft  and  the  woodwork  was  set  in  a  blaze.  The  men  soon  succeeded 
in  putting  out  the  fire,  and  the  church  escaped  with  very  little 
damage. 

Now,  in  this  case  there  was  no  reason  to  suppose  that  the  lightning 


46  LIGHTNING  AND  THUNDER. 

conductor  was  in  any  way  defective.  But  about  half-way  up  the  spire 
there  was,  a  peal  of  eight  bells.  Attached  to  these  bells  were  iron 
wires,  about  the  eighth  of  an  inch  in  diameter,  leading  from  the  clap- 
pers down  to  the  organ-loft,  where  they  came  within  a  short  distance 
of  a  gas-pipe  fixed  in  the  wall.  It  would  seem  that  a  great  part  of 
.the  discharge  was  carried  safely  to  earth  by  the  lightning  conductor. 
But  a  part  branched  off  at  the  bells  in  the  spire,  descended  by  the 
iron  wires,  and  forced  its  way  into  the  organ  loft,  to  reach  the  net- 
work of  gas-pipes,  through  which  it  passed  down  to  the  earth,  melting 
the  soft  leaden  gas-pipe  in  its  course  and  lighting  the  gas. 

The  remedy  foi  this  danger  is  obvious.  All  large  masses  of  metal 
used  in  the  structure  of  a  building — the  leads  and  gutters  of  the  roof, 
the  cast-iron  down-pipes,  the  iron  gas  and  water  mains — should  be 
put  in  good  metallic  connection  with  the  lightning  conductor,  and,  as 
far  as  may  be,  with  one  another.  Connected  in  this  way  they  furnish 
a  continuous  and  effective  line  of  conductors  leading  safely  down  to 
earth  ;  and,  instead  of  being  a  dangerous  rival,  they  become  a  useful 
auxiliary  to  the  lightning  rod. 

I  would  observe,  however,  that  the  lightning  conductor  ought  not 
to  be  connected  directly  with  the  soft  leaden  pipes  which  are  com- 
monly employed  to  convey  gas  and  water  to  the  several  parts  of  a 
building.  Such  pipes,  as  we  have  seen,  are  liable  to  be  melted  when 
any  considerable  part  of  the  lightning  discharge  passes  through  them  ; 
and  thus  much  harm  might  be  done,  and  the  building  might  even  be 
set  on  fire  by  the  lighting  of  the  gas.  Every  good  end  will  be  at- 
tained if  the  conductor  is  put  in  metallic  connection  with  the  iron  gas 
and  water  mains  either  inside  or  outside  the  building. 

Insulation  of  Lightning  Conductors.— It  is  a  question  often 
asked  whether  a  lightning  rod  should  be  insulated  from  the  building 
it  is  intended  to  protect.  I  believe  that  this  practice  was  formerly  rec- 
ommended by  some  writers,  and  I  have  observed  that  glass  insulators 
are  still  employed  not  infrequently  by  builders  in  the  erection  of  light- 
ning conductors  ;  but,  from  the  principles  I  have  set  before  you  to- 
day, it  seems  clear  that  any  insulation  of  this  kind  is,  to  say  the  least, 
altogether  useless.  The  building  to  be  protected  is  itself  in  electrical 
communication  with  the  earth,  and  the  lightning  conductor,  if  effi- 
cient, is  also  in  electrical  communication  with  the  earth — therefore, 
the  lightning  conductor  and  the  building  are  in  electrical  communica- 
tion with  each  other  through  the  earth,  and  any  attempt  at  insulating 
them  from  one  another  above  the  earth  is  only  labor  thrown  away. 

Further,  I  have  just  shown  you  that  the  masses  of  metal  employed 
in  the  structure  or  decoration  of  a  building  ought  to  be  electrically 
connected  with  each  other  and  with  the  lightning  conductor.  Now, 
if  this  be  done,  the  lightning  conductor  is,  by  the  fact,  in  direct  com- 
munication with  the  building,  and  the  glass  insulators  are  utterly 


LIGHTNING  AND  THUNDER.  47 

futile.  Again,  the  building  itself,  during  a  thunderstorm,  becomes 
highly  electrified  by  the  inductive  action  of  the  cloud,  and  needs  to  be 
discharged  through  the  conductor  just  as  the  surrounding  earth  needs 
to  be  discharged  ;  therefore,  the  more  thoroughly  it  is  connected  with 
the  conductor,  the  more  effectively  will  the  conductor  fulfill  its 
functions. 

Personal  Safety  in  a  Thunderstorm.— I  suppose  there  is 
hardly  any  one  to  whom  tjie  question  has  not  occurred,  at  some  time 
or  another,  what  he  had  best  do  to  secure  his  personal  safety  during 
a  thunderstorm.  This  question  is  of  so  much  practical  interest  that  I 
think  I  shall  be  excused  if  I  say  a  few  words  about  it,  though  perhaps, 
strictly  speaking,  it  is  somewhat  beside  the  subject  of  lightning  con- 
ductors. 

At  the  outset,  perhaps,  I  shall  surprise  you  when  I  say  that  you 
would  enjoy  the  most  perfect  security  if  you  were  in  a  chamber  en- 
tirely composed  of  metal  plates,  or  in  a  cage  constructed  of  metal 
bars,  or  if  you  were  incased,  like  the  knights  of  old,  in  a  complete  suit 
of  metal  armor.  This  kind  of  defense  is  looked  upon  as  so  perfect, 
among  scientific  men,  that  Professor  Tait  does  not  hesitate  to  recom- 
mend his  adventurous  young  friends  devoted  to  the  cause  of  science 
to  provide  themselves  with  a  light  suit  of  copper,  and,  thus  protected, 
take  the  first  opportunity  of  plunging  into  a  thundercloud,  there  to 
investigate,  at  its  source,  the  process  by  which  lightning  is  manu- 
factured.1 t 

The  reason  why  a  metal  covering  affords  complete  protection  is 
that,  when  a  conductor  is  electrified,  the  whole  charge  of  electricity 
exists  on  the  outside  surface  of  the  conductor  ;  and  therefore,  when  a 
discharge  takes  place,  it  is  only  the  outside  surface  that  is  affected. 
Thus,  if  you  were  completely  incased  in  a  metal  covering,  and  then 
charged  with  electricity  by  the  inductive  action  of  a  thundercloud,  it 
is  only  the  metal  covering  that  would  undergo  any  change  of  electri- 
cal condition  ;  and  when  the  lightning  flash  would  pass,  it  is  only  the 
metal  covering  that  would  be  discharged. 

Let  me  show  you  a  very  pretty  and  interesting  experiment  to  illus- 
trate this  principle  :  Here  is  a  hollow  brass  cylinder,  open  at  the  ends, 
mounted  on  an  insulating  stand.  On  the  outside  is  erected  a  light 
brass  rod  with  two  pith  balls  suspended  from  it  by  linen  threads.  Two. 
pith  balls  are  also  suspended  by  linen  threads  from  the  inner  surface 
of  the  cylinder.  You  know  that  these  pith  balls  will  indicate  to  us  the 
electrical  condition  of  the  surfaces  to  which  they  are  attached.  If  the 
surface  be  electrified,  the  pith  balls  attached  to  it  will  share  in  its  elec- 
trical condition,  and  will  repel  each  other  ;  if  the  surface  be  neutral, 
the  pith  balls  attached  to  it  will  be  neutral,  and  will  remain  at  rest. 

1  Lecture  on  Thunderstorms,  Nature,  vol.  xxii.,  pp.  365,  437.  See,  also,  a  very  interesting  paper 
by  the  late  Professor  J.  Clerk  Maxwell,  read  before  the  British  Association  at  Glasgow  in  1876,  and 
reprinted  in  the  report  of  the  Lightning  Rod  Conference,  pp.  109,  no. 


48 


LIGHTNING  AND  THUNDER. 


I  now  put  this  apparatus  under  the  influence  of  our  thundercloud, 
that  is,  the  large  brass  conductor  of  our  machine.  The  moment  my 
assistant  turns  the  handle,  the  electricity  begins  to  be  developed  on 
the  conductor,  and  you  see,  at  once,  the  effect  on  the  brass  cylinder. 
The  pith  balls  attached  to  the  outer  surface  fly  asunder  ;  those  at- 
tached to  the  inner  surface  remain  at  rest.  And  now  a  spark  passes  ; 
our  thundercloud  is  discharged  ;  the  inductive  action  ceases  ;  the  pith 
balls  on  the  outside  suddenly  collapse,  while  those  on  the  inside  are 
in  no  way  affected. 

It  is  not  necessary  that  the  brass  cylinder  should  be  insulated.     To 


PROTECTION    FROM   LIGHTNING   FURNISHED   BV  A  CLOSED  CONDUCTOR. 


vary  the  experiment,  I  will  now  connect  it  with  the  earth  by  a  chain  ; 
you  will  observe  that  the  effect  is  precisely  the  same  as  before.  Flash 
after  flash  passes  while  the  machine  continues  in  action  ;  the  outside 
pith  balls  fly  about  violently,  being  charged  and  discharged  alter- 
nately ;  the  inside  pith  balls  remain  all  the  time  at  rest.  Thus  you 
see  clearly  that,  if  you  were  sitting  inside  such  a  metal  chamber  as 
this,  or  covered  with  a  complete  suit  of  metal  armor,  you  would  be 
perfectly  secure  during  a  thunderstorm,  whether  the  chamber  were 
electrically  connected  with  the  earth  or  insulated  from  it. 


LIGHTNING  AND  THUNDER.  49 

Practical  Rules. — But  it  rarely  happens,  when  a  thunderstorm 
comes,  that  an  iron  hut  or  a  complete  suit  of  armor  is  at  hand,  and 
you  will  naturally  ask  me  what  you  ought  to  do  under  ordinary  cir- 
cumstances. First,  let  rne  tell  you  what  you  ought  not  to  do.  You 
ought  not  to  take  shelter  under  a  tree,  or  under  a  haystack,  or  under 
the  lee  of  a  house  ;  you  ought  not  to  stand  on  the  bank  of  a  river,  or 
close  to  a  large  sheet  of  water.  If  indoors,  you  ought  not  to  stay 
near  the  fireplace,  or  near  any  of  the  flues  or  chimneys  ;  you  ought 
not  to  stand  under  a  gasalier  hanging  from  the  ceiling  ;  you  ought 
not  to  remain  close  to  the  gas  pipe's  or  water-pipes,  or  any  large 
masses  of  metal,  whether  used  in  the  construction  of  the  building,  or 
lying  loosely  about. 

The  necessity  for  these  precautions  is  sufficiently  evident  from  the 
principles  I  have  already  put  before  you.  You  want  to  prevent  your 
body  from  becoming  a  link  in  that  broken  chain  of  conductors  which, 
as  we  have  seen,  the  electric  discharge  between  earth  and  cloud  is 
likely  to  follow.  Now  a  tree  is  a  better  conductor  than  the  air  ;  and 
your  body  is  a  betfer  conductor  than  a  tree.  Hence,  the  lightning,  in 
choosing  the  path  of  least  resistance,  would  leave  the  air  to  pass 
through  the  tree,  and  would  leave  the  tree  to  pass  through  you.  A 
like  danger  would  await  you  if  you  stood  under  the  lee  of  a  haystack 
or  of  a  house. 

The  number  of  people  who  lose  their  lives  by  taking  refuge  under 
trees  in  thunderstorms  is  very  remarkable.  As  one  instance  out  of 
many,  I  may  cite  the  following  case  which  was  reported  in  the  Times, 
July  14,  1887  :  "Yesterday  the  funeral  of  a  negress  was  being  con- 
ducted in  a  graveyard  at  Mount  Pleasant,  sixty  miles  north  of  Nash- 
ville, Tennessee,  when  a  storm  came  on,  and  the  crowd  ran  for  shelter 
under  the  trees.  Nine  persons  stood  under  a  large  oak,  which  the 
lightning  struck,  killing  everyone,  including  three  clergymen,  and  the 
mother  and  two  sisters  of  the  girl  who  had  been  buried." 

Again,  every  -large  sheet  of  water  constitutes  practically  a  great 
conductor,  which  offers  a  very  perfect  medium  of  discharge  between 
the  earth  round  about  and  the  cloud.  Therefore,  when  a  thunder- 
cloud is  overhead,  the  sheet  of  water  is  likely  to  become  one  end  of 
the  line  of  the  lightning  discharge  ;  and  if  you  be  standing  near  it, 
the  line  of  discharge  may  pass  through  your  body. 

When  lightning  strikes  a  building,  it  is  very  apt  to  use  the  stack  of 
chimneys  in  making  its  way  to  earth,  partly  because  the  stack  of 
chimneys  is  generally  the  most  prominent  part  of  the  building,  and 
partly  because,  on  account  of  the  heated  air  and  the  soot  within  the 
chimney,  it  is  usually  a  moderately  good  conductor.  Therefore,  if 
you  be  indoors,  you  must  keep  well  away  from  the  chimneys  ;  and  for 
a  similar  reason,  you  must  keep  as  far  as  you  can  from  large  masses 
of  metal  of  every  kind. 


50  LIGHTNING  AND  THUNDER. 

Having  pointed  out  the  sources  of  danger  which  you  must  try  to 
avoid  in  a  thunderstorm,  I  have  nearly  exhausted  all  the  practical  ad- 
vice that  I  have  at  my  command.  But  there  are  some  occasions  on 
which  it  may  be  possible,  not  only  to  avoid  evident  sources  of  dan- 
ger, but  to  make  special  provision  for  your  own  security.  Thus,  for 
example,  in  the  open  country,  if  you  stand  a  short  distance  from  a 
wood,  you  may  consider  yourself  as  practically  protected  by  a  light- 
ning conductor.  For  a  wood,  by  its  numerous  branches  and  leaves, 
favors  very  much  a  quiet  discharge  of  electricity,  thus  tending  to  sup- 
press altogether  the  flash  of  lightning  ;  and  if  the  flash  of  lightning 
does  come,  it  is  much  more  likely  to  strike  the  wood  than  to  strike  you, 
because  the  wood  is  a  far  more  prominent  body,  and  offers,  on  the  whole, 
an  easier  path  to  earth.  In  like  manner,  if  you  place  yourself  near 
a  tall  solitary  tree,  some  twenty  or  thirty  yards  outside  its  longest 
branches,  you  will  be  in  a  position  of  comparative  safety.  If  the  storm 
overtake  you  in  the  open  plain,  far  away  from  trees  and  buildings, 
you  will  be  safer  lying  flat  on  the  ground  than  standing  erect. 

In  an  ordinary  dwelling  house,  the  best  situation  is  probably  the 
middle  story,  and  the  best  position  in  the  room  is  in  the  middle  of  the 
floor  ;  provided,  of  course,  that  there  is  no  gasalier  hanging  from  the 
ceiling  above  or  below  you.  Strictly  speaking,  the  middle  of  the  room 
would  be  a  still  safer  position  than  the  middle  of  the  floor  ;  and  noth- 
ing could  be  more  perfect  than  the  plan  suggested  by  Franklin,  to 
get  into  "a  hammock,  or  swinging  bed,  suspended  by  silk  cords,  and 
equally  distant  from  the  walls  on  every  side,  as  well  as  from  the  ceil- 
ing and  floor,  above  and  below."  An  interesting  case  has  been  re- 
cently recorded,  by  a  resident  of  Venezuela,  which  illustrates  in  a 
remarkable  way  the  excellence  of  this  advice.  "The  lightning,"  he 
says,  "  struck  a  rancho — a  small  country  house,  built  of  wood  and 
mud,  and  thatched  with  straw  or  large  leaves — where  one  man  slept 
in  a  hammock,  another  lay  under  the  hammock  on  the  ground,  and 
three  women  were  busy  about  the  floor  ;  there  were  also  several 
hens  and  a  pig.  The  man  in  the  hammock  did  not  receive  any  in- 
jury whatever,  while  the  other  four  persons  and  the  animals  were 
killed."1 

But,  as  I  can  hardly  hope  that  many  of  you  when  the  thunderstorm 
actually  comes  will  find  yourselves  provided  with  a  hammock,  I  would 
recommend,  as  more  generally  useful,  another  plan  of  Franklin's,  which 
is  simply  to  sit  on  one  chair  in  the  middle  of  the  floor  and  put  your 
feet  up  on  another.  This  arrangement  will  approach  very  nearly  to 
absolute  security  if  you  take  the  further  precaution,  also  mentioned 
by  Franklin,  of  putting  a  feather  bed  or  a  couple  of  hair  mattresses 
under  the  chairs.8 

1  Nature,  vol.  xxxi.,  p.  459. 

9  See  further  information  on  this  interesting  subject  in  the  Report  of  the  Lightning  Rod  Confer- 
ence, pp.  233-5. 


LIGHTNING  AND  THUNDER.  51 

Security  Afforded  by  Lightning  Rods.— You  might,  perhaps, 
be  inclined  to  infer  hastily,  from  the  examples  I  have  set  before  you,  in 
the  course  of  this  lecture,  of  buildings  which  were  struck  and  severely 
injured  by  lightning  though  provided  with  lightning  conductors,  that 
a  lightning  rod  affords  a  very  imperfect  protection  to  life  and  prop- 
erty. But  such  an  idea  would  be  entirely  at  variance  with  the  evi- 
dence at  hand  on  the  subject.  In  all  the  cases  to  which  I  have  re- 
ferred, and  in  many  others  which  might  easily  have  been  cited,  the 
damage  was  done  simply  because  the  lightning  rods  were  deficient  in 
one  or  more  of  the  conditions  on  which  I  have  so  much  insisted. 
Where  these  conditions  are  fulfilled,  the  lightning  flash  will  either  not 
come  down  at  all  upon  the  building,  or,  if  it  do  come,  it  will  be  carried 
harmless  to  the  earth. 

Perhaps  there  is  no  one  fact  that  so  forcibly  brings  home  to  the 
mind  the  complete  protection  afforded  by  lightning  conductors  as  the 
change  which  followed  their  introduction  into  the  Royal  Navy.  I 
have  already  told  you  that  in  former  times  the  damage  done  by  light- 
ning to  ships  of  the  Royal  Navy  was  a  regular  source  of  expenditure, 
amounting  every  year  to  several  thousand  pounds  sterling.  But,  after 
the  general  adoption  of  lightning  conductors  about  forty,  years  ago, 
through  the  indefatigable  exertions  of  Sir  William  Snow  Harris,  this 
source  of  expenditure  absolutely  disappeared,  and  injury  to  life  and 
property  has  long  been  practically  unknown  in  Her  Majesty's  Fleet. 

I  should  say,  however,  that  the  trial  of  lightning  conductors  in  the 
Navy,  though  it  lasted  long  enough  to  prove  their  perfect  efficiency, 
has  almost  come  to  an  end  in  our  own  days.  The  great  iron  monsters 
which  in  recent  times  have  taken  the  place  of  the  wooden  ships  of 
Old  England  are  quite  independent  of  lightning  rods  in  the  common 
sense  of  the  word.  Their  ponderous  masts  are  virtually  lightning 
rods  of  colossal  dimensions,  and  their  unsightly  hulls  are,  so  to  speak, 
earth-plates  of  enormous  size  in  perfect  electrical  contact  with  the 
ocean.  To  add  to  such  structures  lightning  conductors  of  the  com- 
mon kind  would  be  nothing  better  than  "  wasteful  and  ridiculous 
excess." 

As  regards  buildings  on  land,  I  may  refer  to  the  little  province  of 
Schleswig-Holstein,  of  which  I  have  already  spoken  to  you.  From 
some  cause  or  other  this  small  peninsula  is  singularly  exposed  to 
thunderstorms,  and  of  late  years  it  has  been  more  abundantly  pro- 
vided with  lightning  conductors  than,  perhaps,  any  other  district  of 
equal  extent  in  Europe.  Now,  as  a  simple  illustration  of  the  protec- 
tion afforded  by  these  lightning  conductors,  I  may  mention  that,  on 
the  26th  of  May,  1878,  a  violent  thunderstorm  burst  over  the  little 
town  of  Utersen.  Five  several  flashes  of  lightning  fell  in  different 
parts  of  the  town,  but  not  the  slightest  harm  was  done,  each  flash 
being  safely  carried  to  earth  by  a  lightning  conductor.  Further,  it 


52  LIGHTNING  AND  THUNDER. 

appears  from  the  records  of  the  fire  insurance  company  that,  out  of 
552  buildings  injured  by  lightning  during  a  period  of  eight  years — from 
j8yo  to  1878 — only  four  had  lightning  conductors  ;  and  in  these  four 
cases  it  was  found,  on  examination,  that  the  lightning  conductors 
were  defective.1 

It  would  be  easy  to  multiply  evidence  on  this  subject.  But  as  I 
have  already  trespassed,  I  fear,  too  far  on  your  patience,  I  will  con- 
tent myself  with  saying,  in  conclusion,  that  according  to  all  the  high- 
est authorities,  both  practical  and  theoretical,  any  structure  provided 
with  a  lightning  conductor  properly  fitted  up  in  conformity  with  the 
principles  I  have  set  before  you  to-day  is  perfectly  secure  against  light- 
ning. The  lightning,  indeed,  may  fall  upon  it,  but  it  will  pass  harm- 
less to  the  earth  ;  and  the  experience  of  more  than  a  hundred  years 
has  fully  justified  the  simple  and  modest  words  of  the  great  inventor 
of  lightning  conductors  :  "  It  has  pleased  God,  in  His  goodness  to 
mankind,  at  length  to  discover  to  them  the  means  of  securing  their 
habitations  and  other  buildings  from  mischief  by  thunder  and 
lightning." 

NOTE  I. 

ON  THE  LIGHTNING  CONDUCTOR   AT   BEREHAVEN.9 

It  is  satisfactory  to  know  that  the  lightning  conductor  referred  to  in  my  lecture  as 
attached  to  the  lighthouse  at  Berehaven  has  been  put  in  good  order  under  the  best 
scientific  guidance.  The  following  interesting  letter  from  Professor  Tyndall,  which 
appeared  in  the  Times,  August  31,  1887,  gives-the  history  of  the  matter  very  clearly, 
and  fully  bears  out  the  views  put  forward  in  my  lecture  : 

"Your  recent  remarks  on  thunderstorms  and  their  effects  induce  me  to  submit  to 
you  the  following  facts  and  considerations.  Some  years  ago  a  rock  lighthouse  on  the 
coast  of  Ireland  was  struck  and  damaged  by  lightning.  An  engineer  was  sent  down 
to  report  on  the  occurrence  ;  and,  as  I  then  held  the  honorable  and  responsible 
post  of  scientific  adviser  to  the  Trinity  House  and  Board  of  Trade,  the  report  was 
submitted  to  me.  The  lightning  conductor  had  been  carried  down  the  lighthouse 
tower,  its  lower  extremity  being  carefully  embedded  in  a  stone  perforated  to  receive  it. 
If  the  object  had  been  to  invite  the  lightning  to  strike  the  tower,  a  better  arrangement 
could  hardly  have  been  adopted. 

"  I  gave  directions  to  have  the  conductor  immediately  prolonged,  and  to  have  added 
to  it  a  large  terminal  plate  of  copper,  which  was  to  be  completely  submerged  in  the 
sea  The  obvious  convenience  of  a  chain  as  a  prolongation  of  the  conductor  caused 
the  authorities  in  Ireland  to  propose  it  ;  but  I  was  obliged  to  veto  the  adoption  of  the 
chain.  The  contact  of  link  with  link  is  never  perfect.  I  had,  moreover,  beside  me  a 
portion  of  a  chain  cable  through  which  a  lightning  discharge  had  passed,  the  electricity 
in  passing  from  link  to  link  encountering  a  resistance  sufficient  to  enable  it  to  partially 
fuse  the  chain.  The  abolition  of  resistance  is  absolutely  necessary  in  connecting  a 
lightning  conductor  with  the  earth,  and  this  is  done  by  closely  embedding  in  the  earth 
a  plate  of  good  conducting  material  and  of  large  area.  The  largeness  of  area  makes 
atonement  for  the  imperfect  conductivity  of  earth.  The  plate,  in  fact,  constitutes 

1  See  "  Die  Theorie,  die  Anlage,  und  die  Prufung  der  Blitzableiter,"  von  Doctor  W.  Holtz,  Griefs- 
wald,  1878. 

•  See  page  44«  • 


LIGHTNING  AND  THUNDER.  53 

a  wide  door  through  which  the  electricity  passes  freely  into  the  earth,  its  disruptive 
and  damaging  effects  being  thereby  avoided. 

"  These  truths  are  elementary,  but  they  are  often  neglected.  I  watched  with  inter- 
est some  time  ago  the  operation  of  setting  up  a  lightning  conductor  on  the  house  of  a 
neighbor  of  mine  in  the  country.  The  wire  rope  which  formed  part  of  the  conductor 
was  carried  down  the  wall  and  comfortably  laid  in  the  earth  below  without  any  ter- 
minal plate  whatever.  I  expostulated  with  the  man  who  did  the  work,  but  he  obvi- 
ously thought  he  knew  more  about  the  matter  than  I  did.  I  am  credibly  informed 
that  this  is  a  common  way  of  dealing  with  lightning  conductors  by  ignorant  practi- 
tioners, and  the  Bishop  of  Winchester's  palace  at  Farnham  has  been  mentioned  to  me 
as  an  edifice  '  protected '  in  this  fashion.  If  my  informant  be  correct,  the  '  protection ' 
is  a  mockery,  a  delusion,  and  a  snare." 


NOTE  II. 

BOOKS   OF   REFERENCE. 

As  some  of  my  readers  may  wish  to  pursue  the  study  of  lightning  and  light- 
ning conductors  beyond  the  limits  to  which  a  popular  lecture  must,  of  neces- 
sity, be  confined,  I  subjoin  a  list  of  the  books  which  I  think  they  would  be  likely  to 
find  most  useful  for  the  purpose.  Among  ordinary  text-books  on  physics — Jamin, 
Cours  de  Physique,  vol.  i.,  pp.  470-494;  Mascart,  Traite  d'Electricite  Statique,  vol. 
ii.,  pp.  555-579  ;  De  Larive,  A  Treatise  on  Electricity,  in  three  volumes,  London, 
1853-8,  vol.  Hi.,  pp.  90-201;  Daguin,  Traite  de  Physique,  vol.  iii.,  pp.  209-280; 
Riess,  Die  Lehre  von  der  Reibungs-Elektricitat,  vol.  ii.,  pp.  494-564  ;  Miiller-Pouillet, 
Lehrbuch  der  Physik,  Braunschweig,  1881,  vol.  iii.,  pp.  210-225  ;  Scott,  Elementary 
Meteorology,  chap.  x.  Of  the  numerous  special  treatises  and  detached  papers  on  the 
subject,  I  would  recommend  Instruction  sur  les  Paratonnerres  adopte  par  1' Academic 
des  Sciences,  Part  i.,  1823,  Part  ii.,  1854,  Part  iii.,  1867,  Paris,  1874;  Arago,  Sur  le 
Tonnerre,  Paris,  1837  ;  also  his  Meteorological  Essays,  translated  by  Sabine,  London, 
1855  ;  Sir  William  Snow  Harris,  On  the  Nature  of  Thunderstorms,  London,  1843  ; 
also  by  the  same  writer,  A  Treatise  on  Frictional  Electricity,  London,  1867  ;  and 
various  papers  on  lightning  conductors,  from  1822  to  1859  ;  Tomlinson,  The  Thun- 
derstorm, London,  1877  ;  Anderson,  Lightning  Conductors,  London,  1880 ;  Holtz, 
Ueber  die  Theorie,  die  Anlage,  und  die  Priifung  der  Blitzableiter,  Greifswald,  1878  ; 
Weber,  Berichte  ttber  Blitzschlage  in  der  Provinz  Schleswig-Holstein,  Kiel,  1880-1  ; 
Tait,  A  Lecture  on  Thunderstorms,  delivered  in  the  City  Hall,  Glasgow,  in  1880, 
Nature,  vol.  xxii.  ;  Report  of. the  Lightning  Rod  Conference,  London,  1882.  This 
last-mentioned  volume  comes  to  us  with  very  high  authority,  representing,  as  it  does, 
the  joint  labors  of  several  eminent  scientific  men  selected  from  the  following  societies  : 
The  Meteorological  Society,  the  Royal  Institute  of  British  Architects,  the  Society  of 
Telegraph  Engineers  and  Electricians,  the  Physical  Society. 

Since  the  above  was  in  print,  two  lectures  given  before  the  Society  of  Arts  by  Pro- 
fessor Oliver  Lodge,  F.  R.  S.,  have  appeared  in  the  Electrician,  June  and  July,  1888. 
in  which  some  new  views  are  put  forward  respecting  lightning  conductors,  that  seem 
deserving  of  careful  consideration. 


APPENDIX. 


RECENT    CONTROVERSY    ON    LIGHTNING    CONDUCTORS. 

THE  lecture  on  lightning  conductors  contained  in  this  volume 
fairly  represents,  I  think,  the  theory  hitherto  received  on  the 
subject.  It  is,  moreover,  entirely  in  accord  with  the  report  of  the 
Lightning  Rod  Conference,  brought  out  in  1883,  by  a  committee  of 
most  eminent  men,  representing  several  branches  of  science,  who  were 
specially  chosen  to  consider  this  question  some  ten  years  ago. 

Lectures  of  Professor  Lodge. — But,  in  the  month  of  March, 
1888,  two  lectures  were  given  before  the  Society  of  Arts,  in  London, 
by  Professor  Oliver  Lodge,  in  which  this  theory  was  directly  chal- 
lenged, and  attacked  with  cogent  arguments,  supported  by  striking 
and  original  experiments.  These  lectures  gave  rise  to  an  animated 
controversy,  which  culminated  in  a  formal  discussion  at  the  recent 
meeting  of  the  British  Association  in  Bath.  The  discussion  was«car- 
ried  on  with  great  spirit,  and  most  of  the  leading  representatives  of 
physical  and  mechanical  science  took  an  active  part  in  it.  The 
greater  portion  of  this  volume  was  printed  off  before  the  meeting  of 
the  British  Association  took  place.  But  the  discussion  on  the  theory  of 
lightning  conductors  seemed  to  me  so  interesting  and  important  that 
I  thought  it  right,  in  the  form  of  an  Appendix,  to  give  some 'account 
of  the  questions  at  issue,  and  of  the  opinions  expressed  upon  them. 

Professor  Lodge  maintains1  that  the  received  theory  of  lightning 
rods  is  open  to  two  objections.  First,  it  takes  account  only  of  the 
conducting  power  of  the  lightning  rod,  and  takes  no  account  of  the 
phenomenon  known  as  self-induction,  or  electrical  inertia.  Secondly, 
it  assumes  that  the  whole  substance  of  a  lightning  rod  acts  as  a  con- 
ductor, in  all  cases  of  lightning  discharge  ;  whereas  there  is  reason  to 
believe  that,  in  many  cases,  it  is  only  a  thin  outer  shell  that  really 
comes  into  action.  I  will  deal  with  these  two  points  separately. 

The  Effect  of  Self-induction.— When  an  electric  discharge  be- 
gins to  pass  through  a  conductor,  a  momentary  back  electro-motive 
force  is  developed  in  the  conductor,  which  obstructs  its  passage. 
This  phenomenon  is  called  by  some  self-induction,  by  others  electrical 
inertia  ;  but  its  existence  is  admitted  by  all.  Now,  when  a  flash  of 
lightning,  so  to  say,  falls  on  a  lightning  rod,  the  back  electro-motive 

1  See  his  Lectures,  published  in  the  Electrician,  June  22,  June  29,  July  6,  and  July  13, 1888. 


56  APPENDIX. 

force  developed  is  very  considerable  ;  and  it  may  offer  so  great  an 
obstruction  that  the  discharge  will  find  an  easier  passage  by  some  other 
route,  such  as  the  stone  walls  and  woodwork,  and  furniture  of  the 
building. 

According  to  this  view,  the  obstruction  which  a  flash  of  lightning 
encounters  in  a  conductor  consists  partly  of  the  resistance  of  the  con- 
ductor, in  the  ordinary  sense  of  the  word  resistance,  and  partly  of  the 
back  electro-motive  force  due  to  self-induction.  The  sum  of  these 
two  Professor  Lodge  calls  the  impedance  of  the  lightning  rod  ;  and 
he  considers  that  the  impedance  may  be  enormously  great,  even  when 
the  resistance,  in  the  ordinary  sense,  is  comparatively  small. 

In  support  of  this  view  he  has  devised  the  following  extremely  in- 
genious and  remarkable  experiment.  A  large  Leyden  jar,  L,  was  ar- 
ranged in  such  a  manner  that,  while  it  received  a  steady  charge  from 


INDUCTION    EFFECT  OF   LEYDEN  JAR   DISCHARGE. 

M  Electrical  Machine  I      A  B  Air  Spaces  between  Brass  Knobs. 

L  Leyden  Jar.  |      C      Conducting  Wire. 

an  electrical  machine,  it  discharged  itself,  at  intervals,  across  the  air 
space  at  A,  between  two  brass  balls.  The  discharge  had  then  two 
alternative  paths  before  it ;  one  through  a  conducting  wire,  C,  the 
other  across  a  second  air  space,  between  two  brass  balls  at  B.  During 
the  experiment,  the  two  balls  at  A  were  kept  at  a  fixed  distance  of 
one  inch  apart ;  but  the  distance  between  the  two  balls  at  B  was 
varied.  The  conductor,  C,  used  in  the  first  instance,  was  a  stout  cop- 
per wire,  about  forty  feet  long,  and  having  a  resistance  of  only  one- 
fortieth  of  an  ohm. 

It  was  found  that,  so  long  as  the  distance  between  the  B  knobs  was 
less  than  1.43  inches,  all  the  discharges  passed  across  between  the 


CONTROVERSY  ON  LIGHTNING  CONDUCTORS.    57 

knobs,  in  the  form  of  a  spark.  When  the  distance  exceeded  1.43 
inches,  all  the  discharges  passed  through  the  conductor,  C,  and  no 
spark  appeared  between  the  balls  at  B.  And  when  the  distance  was 
exactly  1.43  inches,  the  discharge  sometimes  took  place  between  the 
knobs,  and  sometimes  followed  the  conductor,  C.  The  interpretation 
given  to  these  facts  is  that  the  obstruction  offered  by  the  conductor  C 
was  about  equal  to  the  resistance  of  1.43  inches  of  air  ;  and  it  is  pro- 
posed to  call  this  distance,  under  the  conditions  of  the  experiment, 
the  critical  distance. 

Coming  now  to  the  application  of  these  results,  Professor  Lodge 
argues  that  the  conductor  C,-in  his  experiment,  represents  a  lightning 
rod  of  unimpeachable  excellence  ;  and  yet,  in  certain  cases,  the  dis- 
charge refuses  to  follow  the  conductor,  and  prefers  to  leap  across  a 
considerable  space  of  air,  notwithstanding  the  enormous  resistance  it 
there  encounters.  In  like  manner,  he  says,  a  flash  of  lightning  may, 
in  certain  cases,  leave  a  lightning  rod  fitted  up  in  the  most  orthodox 
manner,  and  force  its  way  to  earth  through  resisting  masses  of  mason 
work  and  such  chance  conductors  as  may  come  across  its  path. 

This  conclusion,  he  admits,  is  altogether  at  variance  with  the  re- 
ceived views  on  the  subject  ;  but  he  contends  that  it  is  perfectly  in 
accord  with  the  scientific  theory  of  an  electrical  discharge.  The  mo- 
ment the  discharge  begins  to  pass  in  the  conductor,  it  encounters  the 
obstruction  due  to  self-induction  ;  and  this  obstruction  is  so  great  that 
the  bad  conductors  offer,  on  the  whole,  an  easier  path  to  earth. 

Variation  of  the  Experiment. — When  the  experiment  was  va- 
ried by  substituting  a  thin  iron  wire  for  the  stout  copper  wire  at  first 
employed,  a  very  curious  result  was  obtained.  The  wire  chosen  was 
of  the  same  length  as  the  copper,  but  had  a  resistance  about  1,300 
times  as  great ;  its  resistance  being,  in  fact,  33.3  ohms.  Nevertheless, 
in  this  experiment,  when  the  B  knobs  were  at  a  distance  of  1.43  inches, 
no  spark  passed,  which  showed  that  the  discharge  always  followed 
the  line  of  the  conductor,  and  therefore  that  the  conductor  offered 
less  obstruction  than  1.43  inches  of  air.  The  knobs  were  then  brought 
gradually  nearer  and  nearer  ;  and  it  was  not  until  the  distance  was  con- 
siderably reduced  that  the  sparks  began  to  pass  between  them.  When 
the  distance  was  exactly  1.03  inches,  the  discharge  sometimes  passed 
between  the  knobs,  and  sometimes  through  the  conductor  ;  this  was, 
therefore,  the  critical  distance,  in  the  case  of  the  iron  wire.  Thus  it 
appeared  that  the  obstruction  offered  to  the  discharge  by  the  iron 
wire  was  much  less  than  that  offered  by  the  copper,  the  one  being 
equal  to  a  resistance  of  only  1.03  inches  of  air,  the  other  to  a  resist- 
ance of  1.43  inches. 

It  does  not  appear  that  Professor  Lodge  undertakes  to  offer  any 
satisfactory  explanation  of  this  result.  He  has  come  to  the  conclu- 
sion, from  his  various  experiments,  that,  in  the  case  of  a  sudden 


58  APPENDIX. 

discharge,  difference  of  conducting  power  between  fairly  good  con- 
ductors is  a  matter  of  practically  no  account ;  and  that  difference  of 
sectional  area  is  a  matter  of  only  trifling  account.  But  he  does  not 
see  why  a  thin  iron  wire  should  have  a  smaller  impedance  than  a 
much  thicker  wire  of  copper.  He  proposes  to  repeat  the  experiments 
so  as  to  confirm  or  to  modify  the  result,  which  for  the  present  seems 
to  him  anomalous.1 

The  Outer  Shell  only  of  a  Lightning  Rod  Acts  as  a  Con- 
ductor.— As  a  consequence  of  self-induction  or  electrical  inertia, 
Professor  Lodge  contends  that  a  lightning  discharge  in  a  conductor 
consists  of  a  series  of  oscillations.  These  oscillations  follow  one  an- 
another  with  extraordinary  rapidity — there  may  be  a  hundred  thou- 
sand in  a  second,  there  may  be  a  million.  Now  it  has  been  shown 
that,  when  a  current  starts  in  a  conductor,  it  does  not  start  at  once  all 
through  its  section  ;  it  begins  or.  the  outside,  and  then  gradually,  but 
rapidly,  penetrates  to  the  interior.  From  this  he  infers  that  the  ex- 
tremely rapid  oscillations  of  a  lightning  discharge  have  not  time  to 
penetrate  to  the  interior  of  a  conductor.  The  electricity  keeps  surg- 
ing to  and  fro  in  the  superficial  layer  or  outer  shell,  while  the  interior 
substance  of  the  rod  remains  inert  and  takes  no  part  in  the  action.  A 
conductor,  therefore,  will  be  most  efficient  for  carrying  off  a  flash  of 
lightning  if  it  present  the  greatest  possible  amount  of  surface ;  a  thin, 
flat  tape  will  be  more  efficient  wthan  a  rod  of  the  same  mass  ;  and  a 
number  of  detached  wires  more  efficient  than  a  solid  cylinder.  As  for 
existing  lightning  conductors,  the  greater  part  of  their  mass  would, 
in  many  cases,  have  no  efficacy  whatever  in  carrying  off  a  flash  of 
lightning. 

The  Discussion. — The  discussion  at  the  meeting  of  the  British 
Association  was  opened  by  Mr.  William  H.  Preece,  F.R.S.,  Electrician 
to  the  Post  Office,  who  claimed  to  have  500,000  lightning  conductors 
under  his  control.  He  expressed  his  conviction  that  a  lightning  rod, 
properly  erected  and  duly  maintained,  was  a  perfect  protection  against 
injury  from  lightning ;  and  in  support  of  this  conviction  he  urged  very 
strongly  the  report  of  the  Lightning  Rod  Conference.  This  report 
represented  the  mature  judgment  of  the  most  eminent  scientific  men, 
who  had  devoted  years  to  the  study  of  the  question  ;  and  he  wished 
particularly  to  bring  before  the  meeting  their  clerrand  decisive  asser- 
tion— an  assertion  he  was  there  to  defend — that  -'there  is  no  authen- 
tic case  on  record  where  a  properly  constructed  conductor  failed  to  do 
its  duty." 

The  new  views  put  forward  by  Professor  Lodge  were  based,  in 
great  measure,  on  his  theory  that  a  lightning  discharge  consisted  of  a 
series  of  rapid  oscillations.  But  this  theory  should  be  received  with 
great  caution.  It  seemed  to  be  nothing  more  than  a  deduction  from 

1  See  paper  read  at  the  meeting  of  the  British  Association,  in  Bath,  1888,  published  in  the  Elec- 
trician^ page  607.  September  14. 


CONTROVERSY  ON  LIGHTNING  CONDUCTORS.     59 

certain  mathematical  formulas,  and  was  not  supported  by  any  solid 
basis  of  observation  or  experiment.  Besides,  there  were  many  facts 
against  it.  They  all  knew  that  a  flash  of  lightning  magnetized  steel 
bars,  deranged  the  compasses  of  ships  at  sea,  and  transmitted  signals 
on  telegraph  wires.  But  such  effects  could  not  be  produced  by  a  series 
of  oscillations,  which,  being  equal  and  opposite,  would  neutralize  each 
other.  It  was  alleged  that  these  rapid  oscillations  occurred  in  the 
discharge  of  a  Leyden  jar.  That  might  be  true,  and  probably  was 
true  ;  but  they  were  not  dealing  with  Leyden  jars,  they  were  dealing 
with  flashes  of  lightning.  If  there  was  any  analogy  between  the  dis- 
charge of  a  Leyden  jar  and  a  flash  of  lightning,  it  was  to  be  found, 
not  in  the  external  discharge  employed  by  Professor  Lodge  in  his  ex- 
periments, but  in  the  bursting  of  the  glass  cylinder  between  the  two 
coatings  of  the  jar. 

Lord  Rayleigh  thought  the  experiments  of  Professor  Lodge  were 
likely  to  have  important  practical  applications  to  lightning  conductors. 
But  though  these  experiments  were  valuable  as  suggestions,  they  did 
not  furnish  a  sufficient  ground  for  adopting  any  new  system  of  pro- 
tection. It  was  only  by  experience  with  lightning  conductors  them- 
selves that  the  question  could  be  finally  settled. 

Sir  William  Thomson  hoped  for  great  fruit  from  the  further  investiga- 
tion of  self-induction  in  the  case  of  sudden  electrical  discharges.  He 
warmly  encouraged  Professor  Lodge  to  continue  his  researches  ;  but 
he  expressed  no  decided  opinion  on  the  question  at  issue.  Incident- 
ally he  observed  that  the  best  security  for  a  gun-powder  magazine 
was  an  iron  house  ;  no  lightning  conductor  at  all,  but  an  iron  roof, 
iron  walls,  and  an  iron  floor.  Wooden  boards  should,  of  course,  be 
placed  over  the  floor  to  prevent  the  danger  of  sparks  from  people 
walking  on  sheet-iron.  This  iron  magazine  might  be  placed  on  a  dry 
granite  rock,  or  on  wet  ground  ;  it  might  even  be  placed  on  a  founda- 
tion under  water ;  it  might  be  placed  anywhere  they  pleased ;  no 
matter  what  the  surroundings  were,  the  interior  would  be  safe.  He 
thought  that  was  an  important  practical  conclusion  which  might  safely 
be  drawn  from  the  consideration  of  these  electrical  oscillations  and 
the  experiments  regarding  them. 

Professor  Rowland,  of  the  Johns  Hopkins  University,  America,  said 
that  the  question  seemed  to  be  whether  the  experiment  of  Professor 
Lodge  actually  represented  the  case  of  lightning.  He  was  very  much 
disposed  to  think  it  did  not.  In  the  experiment  almost  the  whole  cir- 
cuit consisted  of  good  conductors  ;  whereas,  in  the  case  of  lightning, 
the  path  of  the  discharge  was,  for  the  most  part,  through  the  air,  and 
therefore  it  might  be  an  entirely  different  phenomenon.  The  air  be- 
ing a  very  bad  conductor,  a  flash  of  lightning  might,  perhaps,  not 
consist  of  oscillations,  but  rather  of  a  .single  swing.  Moreover,  it  was 
not  at  all  clear  that  the  lejigth  of  the  spark,  in  the  experiment,  could 


60  APPENDIX. 

be  taken  as  a  measure  of  the  obstruction  offered  by  the  conductor. 
Professor  George  Forbes  was  greatly  impressed  with  the  beauty  and 
significance  of  Professor  Lodge's  experiments,  but  he  did  not  think 
the  result  so  clear  that  they  should  be  warranted  in  abandoning  the 
principles  laid  down  by  the  Lightning  Rod  Conference. 

M.  de  Fonvielle,  of  Paris,  supported  the  views  of  Mr.  Preece.  He 
cited  the  example  of  Paris,  where  they  had  erected  a  sufficient  num- 
ber of  lightning  conductors,  according  to  the  received  principles,  and 
calamities  from  lightning  were  practically  unknown.  He  suggested 
that  the  Eiffel  Tower,  which  they  were  now  building,  and  which  would 
be  raised  to  the  height  of  a  thousand  feet,  would  furnish  an  unrivalled 
opportunity  for  experiments  on  lightning  conductors. 

Sir  James  Douglass,  Chief  Engineer  to  the  Corporation  of  Trinity 
House,  had  a  large  experience  with  lighthouse  towers.  The  light- 
ning rods  on  these  towers  had  been  erected  and  maintained  during 
the  last  fifty  years  entirely  according  to  the  advice  of  Faraday.  They 
never  had  a  serious  accident ;  and  such  minor  accidents  as  did  occur 
from  time  to  time  were  always  traced  to  some  defect  in  the  con- 
ductor. They  had  now  established  a  more  rigid  system  of  inspection, 
and  he,  for  one,  should  feel  perfectly  safe  in  any  tower  where  this 
system  was  carried  out. 

Mr.  Symons,  F.R.S.,  Secretary  to  the  Meteorological  Society,  had 
taken  part  in  a  discussion  on  lightning  conductors  as  long  ago  as  1859. 
It  had  been  a  hobby  with  him  all  his  life  to  investigate  the  circum- 
stances of  every  case  he  came  across  in  which  damage  was  done  by 
lightning,  and  the  general  impression  left  by  his  investigations  entirely 
coincided  with  the  views  just  expressed  by  Sir  James  Douglass.  He 
had  been  a  member  of  the  Lightning  Rod  Conference,  and  was  the 
editor  of  their  report ;  and  he  wished  to  enter  his  protest  against  the 
idea  of  rejecting  all  that  had  hitherto  been  done  in  connection  with 
lightning  conductors  on  the  strength  of  mere  laboratory  experiments. 

Professor  Lodge,  in  reply,  said  he  could  perfectly  understand  the 
position  of  those  who  held  that  a  lightning  rod  properly  fitted  up 
never  failed  to  do  its  duty,  because,  whenever  it  failed,  they  said  it 
was  not  properly  fitted  up.  The  great  resource  in  such  cases  was  to 
ascribe  the  failure  to  bad  earth  contact.  He  thought  a  good  earth 
contact  was  a  very  good  thing,  but  he  could  not  understand  why  such 
extraordinary  importance  should  be  attached  to  it.  A  lightning  rod 
had  two  ends — an  earth  end  and  a  sky  end — and  he  did  not  see  why 
good  contact  was  more  necessary  at  one  end  than  at  the  other.  If  a 
few  sharp  points  sticking  out  from  the  conductor  were  sufficient  for  a 
good  sky  contact,  why  were  they  not  sufficient  also  for  a  good  earth 
contact  ? 

Besides,  though  a  bad  earth  contact  might  explain  why  a  certain 
amount  of  disruption  should  take  place  at  the  earth  where  the  bad 


CONTROVERSY  ON  LIGHTNING  CONDUCTORS.     61 

contact  existed,  he  did  not  see  how  it  accounted  for  the  flash  shooting 
off  sideways  half-way  down  the  conductor.  Again,  what  does  a  baci 
earth  contact  mean  ?  If  an  electrical  engineer  finds  a  resistance  of  a 
hundred  ohms,  he  will  rightly  pronounce  the  earth  contact  to  be  very 
bad  indeed.  But  why  should  the  lightning  flash  leave  a  conductor 
with  a  resistance  of  a  hundred  ohms  in  order  to  follow  a  line  of  non- 
conductors where  it  encounters  a  resistance  of  many  thousand 
ohms  ? 

He  accepted  the  statement  of  Mr.  Preece  that  his  whole  theory  de- 
pended on  the  existence  of  oscillations  in  the  lightning  discharge  ; 
but  there  was  good  reason  to  believe  they  existed,  because  they  were 
proved  to  exist  in  the  discharge  of  a  Leyden  jar.  Mr.  Preece  objected 
that  an  oscillating  discharge  could  not  produce  magnetic  effects,  as  a 
flash  of  lightning  was  known  to  do.  He  confessed  he  was  unable  to 
explain  how  an  oscillating  discharge  produced  such  effects; '  but  that 
it  could  produce  them  there  was  no  doubt  whatever,  for  the  discharge 
of  a  Leyden  jar  produces  magnetic  effects,  and  we  have  ocular  dem- 
onstration that  the  discharge  of  a  Leyden  jar  is  an  oscillating  dis- 
charge. 

As  to  the  assurances  we  had  received  from  electrical  engineers  that 
a  properly  fitted  lightning  conductor  never  fails,  he  should  like  to  ask 
them  how  the  Hotel  de  Ville,  in  Brussels,  had  been  set  on  fire  by 
lightning  on  the  ist  of  last  June.  The  system  of  lightning  conduct- 
ors on  this  building  had  been  erected  in  accordance  with  the  received 
theory,  and  had  been  held  up  by  writers  on  the  subject  as  the  most 
perfect  in  Europe.  Unless  some  explanation  were  forthcoming  to 
account  for  its  failure,  we  could  no  longer  regard  lightning  conductors 
as  a  perfect  security  against  danger. 

The  President  of  Section  A,  Professor  Fitzgerald,  in  bringing  the 
discussion  to  a  close,  observed  that  one  result  of  this  meeting  would 
be  to  give  a  new  interest  to  the  phenomena  of  static  electricity  and  its 
practical  applications.  He  was  inclined  himself  to  think  that  the  ex- 
periments of  Professor  Lodge  were  not  quite  analogous  to  the  case  of 
a  flash  of  lightning.  In  comparing  the  discharge  of  a  Leyden  jar 
with  a  flash  of  lightning  they  should  look  for  the  analogy,  not  so 
much  in  the  external  discharge  through  a  series  of  conductors,  but 
rather,  as  Mr.  Preece  had  observed,  in  the  bursting  of  the  glass  be- 
tween the  two  coatings  of  the  jar.  As  regarded  the  oscillations  in  a 
Leyden  jar  discharge,  he  did  not  think  such  oscillations  were  at  all 
necessary  to  account  for  the  phenomena  observed  in  the  experiments. 
Many  of  the  results  which  Professor  Lodge  seemed  to  think  would 
require  some  millions  of  oscillations  per  second  would  be  produced  by 
a  single  discharge  lasting  for  a  millionth  of  a  second.  Improvements, 
perhaps,  were  possible  in  our  present  system  of  lightning  conductors, 

1  See  a  very  ingenious  hypothesis,  to  account  for  this  phenomenon,  suggested  by  Professor  Ewing  in 
the  Electrician^  p.  712.     October  5,  1888. 


62  APPENDIX. 

but  practical  experience  had  shown,  however  we  might  reason  on  the 
matter,  that,  on  the  whole,  lightning  conductors  had  been  a  great 
protection  to  mankind  from  the  dangers  of  lightning. 

Summary. — I  will  now  try  to  sum  up  the  results  of  this  interesting 
discussion,  and  state  briefly  the  conclusions  which,  as  it  seems  to  me, 
may  be  deduced  from  it.  First,  I  would  remind  my  readers  that  a 
lightning  rod  has  two  functions  to  fulfill.  Its  first  function  is  to  pro- 
mote a  gradual,  but  rapid,  discharge  of  electricity  according  as  it  is 
developed,  and  thus  to  prevent  such  an  accumulation  as  would  lead  to 
a  flash  of  lightning.  Its  second  function  is  to  convey  the  flash  of 
lightning,  when  it  does  come,  harmless  to  the  earth.  Now,  the  new 
views  advanced  by  Professor  Lodge  in  no  way  impugn  the  efficiency 
of  lightning  rods  as  regards  their  first  function  ;  and  it  is  evident  that 
the  greater  the  number  of  lightning  rods  distributed  over  a  given  area, 
the  more  perfectly  will  this  function  be  fulfilled.  This  is  a  point  of 
great  practical  importance  which  seemed  to  me,  in  some  degree,  lost 
sight  of  during  the  progress  of  the  discussion. 

Secondly,  it  was  practically  admitted  by  the  highest  authorities  that 
the  experiments  and  reasoning  of  Professor  Lodge  afford  good  grounds 
for  reconsidering  the  received  theory  of  lightning  conductors  as  re- 
gards their  second  function — that  of  carrying  the  lightning  flash 
harmless  to  the  earth.  •  But  there  was  undoubtedly  a  general  feeling 
that  it  would  be  rash  to  set  aside,  all  at  once,  the  received  theory  on 
the  strength  of  laboratory  experiments  made  under  conditions  widely 
different  from  those  which  actually  exist  in  a  lightning  discharge. 
Experiments  are  wanted  on  a  larger  scale  ;  and,  if  possible,  experi- 
ments with  lightning  rods  themselves. 

Thirdly,  the  testimony  of  electrical  engineers  who  have  had  large 
experience  with  lightning  conductors  seems  almost  unanimous  that  a 
lightning  conductor  erected  and  maintained  in  accordance  with  the 
conditions  prescribed  by  the  Lightning  Rod  Conference  gives  perfect 
protection.  It  was  certainly  unfortunate  that  the  Hotel  de  Ville,  in 
Brussels,  which  was  reputed  the  best  protected  building  in  Europe, 
should  have  been  damaged  by  lightning  just  two  months  before  the 
discussion  took  place  ;  but  no  certain  conclusion  can  be  drawn  from 
this  catastrophe  until  we  know  exactly  the  conditions  under  which  it 
occurred. 

So  the  matter  stands,  awaiting  further  investigation. 


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No.  40.    The  Evidence  of  Organic  Evolution. 

No.  17.    Progress:   Its  Law  and  Cause.    With 
other  disquisitions.  By  Herbert  Spencer. 

By  George  J,  Romanes.  F.R.S. 
No.  41.    Current  Discussions  in  Science.    By 

W.  M.  Williams,  F.C.S. 

No.  18.    Lessons  in  Electricity,    (sixty   illustra- 
tions).   By  John  Tyndall,  F.R.S. 

No.  42.    History  of  the  Science  of  Politic*. 

By  Frederick  Pollock. 

No.  19.    Familiar  Essays    on  Scientific  Sub- 

No. 43.    Darwin    and    Humboldt.     By    Prof. 

jects.    By  Richard  A.  Proctor. 

Huxley,  Prof.  Agassiz,  and  others. 

No.  20.    The  Romance  of  Astronomy.    By  R. 

No.  44.    The  Dawn  of  History.    Parti.    ByO, 

Kalley  Miller,  M.A. 

F.  Keary,  of  the  British  Museum. 

Ho.  21.    The  Physical  Basis  of  Life,  with  other 

No.  45.    The  Dawn  of  History.    Part  II. 

essays.    By  Thomas  H.  Huxley,  F.R.S. 

No.  46.    The    Diseases    of   Memory.      By    Th. 

Ilibet.    Translated  from  the  French  by 
J.  Fitzgerald,  M.A. 


No.  47.    The    Childhood     of     Religion. 

Edward  Clodd,  F.R.A.8. 


By 


No.  48.  Life  in  Nature.  (Illustrated.)  By  James 
Hinton. 

No.  49.  The  Sun ;  its  Constitution,  its  Phenom- 
ena, its  Condition.  By  Judge  Nathan  T. 
Carr,  Columbus.  Ind. 

No.  60.  Money  and  the  Mechanism  of  Ex- 
change. Part  I.  By  Prof.  W.  Stanley 
Jevons,  F.R.S. 

No.  51.  Money  and  the  Mechanism  of  Ex- 
change. Part  II. 

No.  52.    The  Diseases  of   the    Will.     By  Th. 

Ribot.    Translated  from  the  French  by 
J.  Fitzgerald. 

No.  53.  Animal  Automatism,  and  other  Essays, 
By  Prof.  T.  H.  Huxley.  F.R.S. 

No.  54.    The  Birth  and  Growth  of  Myth.    By 

Edward  Clodd.  F.R.A.8. 

No.  56.    The  Scientific  Basis  of  Morals,  and 

other  Essays.  By  William  Kiufigdom  Clif- 
ford, F.R.S. 
Illusions.    Part  I.    By  James  Sully. 


No.  66. 
No.  57. 
No.  58. 


Illusions.    Part  II. 

The  Origin  of  Species.    (Double  num- 
ber).   Part  I.    By  Charles  Darwin. 
No.  59.    The  Origin  of  Species.    Double  num- 
ber.   Part  II. 


No.  60.    The    Childhood    of   the   World. 

Edward  Clodd. 


•r 


No.  61.  Miscellaneous  Essays.  By  Richard  A. 
Proctor. 

No.  62    The  Religions  of  the  Ancient  World. 

By  Prof.  Geo.  Rawlinson,  Univ.  of  Oxford, 
(Doable  Number). 

No.  63.  Progressive  Morality.  By  Thomas 
Fowler,  LL.D.,  President  of  Corpus 
Christ!  Coll.,  Oxford. 

No.  64.  The  Distribution  of  Animals  and 
Plants.  By  A.  Russell  Wallace  and  W. 

T.  Thist  In  -  .11  Dyer. 

No.  65.    Conditions  of  Mental  Development: 

and  other  essays.    By  William  Kingdon 
Clifford. 

No.  66.  Technical  Education :  and  other  essays. 
By  Thomas  H.  Huxley,  F.R.S. 

No.  67.  The  Black  Death.  An  account  of  the 
Great  Pestilence  of  the  14th  Century. 
By  J.  F.  C.  Hecker,  M.  D. 

No.  68.  Three  Essays.  By  Herbert  Spencer. 
Special  Number. 

No.  69.  Fetichism:  A  Contribution  to  Anthropo- 
logy and  the  History  of  Religion.  By 
Fritz  Schultze,  Ph.D.  Double  number. 

No.  70.  Essays  Speculative  and  Practical. 
By  Herbert  Spencer. 

No.  71.  Anthropology.  By  Daniel  Wilson,  Ph. 
D.  With  Appendix  on  Archaeology.  By 
E.B.  Tylor,F.B.S. 

No  72  The  Dancing  Mania  of  the  Middle 
Ages.  By  J.  F.  C.  Hecker,  M.D. 


Wo.  73.  Evolution  in  History,  Language  :m<: 
Science.  Four  Addresses  delivered  a. 
the  London  Crystal  Palace  School  of  Art 
Science  and  Literature. 

No.  74. ]  The  Descent  of  Man,  and  Selection  it 
No.  76.  I  Relation  to  Sex.  (Numerous  IUu»tration»\ 
No.  76.  f  By  Charles  Darwin.  Nos.  74.  7;">,  76  an. 
No.  77.  J  single  Not.;  No.  77.  is  a  double  No. 

No.  78.  Historical  Sketch  of  the  Distribu 
tion  of  Land  in  England.  By  Wi] 
liam  Lloyd  Birkbeck,  M.A. 

No.  79.  Scientific  Aspect  of  some  Familia 
Tilings.  By  W.  M.  Williams. 

No.  80.  Charles  Darwin.  His  Life  and  Work 
By  Grant  Allen.  (Double  number). 

No.  81.  The  Mystery  of  Matter,  and  th«< 
Philosophy  of  Ignorance.  Two  EE 

says  by  J.  Allansoa  Picton. 

No.  82.  Illusions  of  the  Senses:  and  other  Et 
says.  By  Richard  A.  Proctor. 

No,  83.  Profit-Sharing  Between  Capital  an* 
Labor.  Six  Essays.  By  Sedley  Tayloi 

No.  84,    Studies'  of  Animated  Nature.    Fou 

Essays  on  Natural  History.      By  W.  t 
Dallas,  F.L.S. 

No.  86.    The  Essential  Nature    of    Religion 

By  J.  Alhin.-ou  Picton. 

No.  86.  The  Unseen  Universe,  and  tho  Philosc 
phy  of  the  run- Sciences.  By  Prof.  Wn; 
£ingdon  Clifford,  F.R.8. 

No.  87.    The  Morphine  Habit.    By  Dr.  B.  Bal 

of  the  Paris  Faculty  of  Medicine. 

No.  88.  Scienoe  and  Crime  and  other  Essayi 
By  Andrew  Wilson.  F.R.8.E. 

No.  89.  The  Genesis  of  Scienoe.  By  Herbei 
Spencer. 

No.  90.  Notes  on  Earthquakes:  with  Fourier 
Miscellaneous  Lssays.  By  Richard  / 
Proctor. 

No.  91.  The  Rise  of  Universities.  By  S.  ! 
Laurie,  LL.D.  (Double  number). 

No.  92.  The  Foiinatioii  of  Vegetable  Moul 
through  the  Action  of  Eart 
Worms.  By  Charles  Darwin,  LL.l 
F.R.S.  (Double  number). 

No.  93.  Scientific  Methods  of  Capital  Put 
ishment.  By  J.  Mount  Bleyer,  .Y.i 
(Special  number). 

No.  94.    The  Factors  of  Organic  Evolution 

By  Herbert  Spencer. 

No.  95.    The  Diseases  of  Personality.    By  T 

Ribot.    Translated  from  the  French  1 
J.  Fitzgerald,  M.A. 

No.  96.    A  Half-Century  of  Science.    By  Pr< 

Thomas  H.  Huxley,  and  Grant  Allen. 

No.  97.    The  Pleasures  of  Life.    By  Sir  Jol 

Lubbock,  Bart. 

No.  98.  Cosmic  Emotion:  Also  the  Teaol 
ings  of  Science.  By  William  Kingd< 
Clifford.  (Special  number). 

No.  93.  Nature  Studies.  By  Prof.  F.  R.  Eat 
Lowe ;  Dr.  Robert  Brown,  F.L.H  •  G» 
G.  Chisholm,  F.R.G.S.,  and  James  D 
las,  F.L.S. 


o.  iw.  science  nricl  roetr7/,  witli  otlier  Es- 

says.   By  Andrew  Wilson,  F.R.S.E. 
o.  101.  ^Esthetics;  Dreams  and  Association 

of  Ideas.     By    Jus.    Sully    and    Geo. 

Croom  Robertson. 
o.  102.  Ultimate  Finance;    A  True  Theory 

of  Co-operation.    By  William  Nelson 

Black. 
o.  103.  The   Commg  Slavery;    The  Sins  of 

Legislators;  The  Great    Political 

Superstition.    By  Herbert  Spencer. 
o.  104.  Tropical    Africa,     By    Henry    Drum- 

mond,  F.E.S. 
o.  105.  Freedom    in  Science  and  Teaching. 

By  Ernst  Haeckel,  of  the  University  of 

Jena.    With  a  Prefatory  Note  by  Prof. 

Huxley. 

o.  106.  Force    and    Energy.     A  Theory  of 
Dynamics.    By  Grant  Allen. 


o.  107. 
b.  108. 

o.  109. 
b.  110. 

o.  111. 
b.  112. 

b.  113. 


No.  121.  Utilitarianism.  By  John  Stuart  M11L 
No.  122.  Upon  the  Origin  of  Alpine  and 
Italian  Lakes  and  upon  Glacial  Ero- 
sion. Maps  and  Illustrations.  By  Ram- 
sey, Ball,  Murchison,  Studer,  Favre, 
Whymper  and  Spencer.  Part  I.  (Double 
Number.) 


No.  123. 


Upon    the    Origin    of    Alpine 
Italian  Lakes,  Etc.,  Etc.    Part  II. 


a«d 


By 


By  Thomas 


fo.114. 


Ultimate  Finance.  A  True  Theory 
of  Wealth.  By  William  Nelsou 
Black. 

English,  Past  and  Present.  Part.  I. 
By  Richard  Chenevix  Trench,  (Double 
number). 

English,  Past  and  Present.  Part  II. 
By  Richard  Chenevix  Trench. 

The  Story  of  Creation.  A  Plain  Ac- 
count of  Evolution.  By  Edward 
Clodd.  (Double  number). 

The  Pleasures  of  Life.  Part  II.  By  Sir 
John  Lubbock,  Bart. 

Psychology  of  Attention.  By  Th. 
ttibot.  Translated  from  the  French  by  J. 
Fitzgerald,  M.A. 

Hypnotism.  Its  History  and  Develop- 
ment. By  Fredrik  Bjornstrom,  M.D., 
Head  Physician  of  the  Stockholm  Hospi- 
tal, Professor  of  Psychiatry.  Late  Royal 
Swedish  Medical  Councillor.  Authorized 
Translation  from  the  Second  Swedish 
Edition  by  Baron  Nils  Posse,  M.G.,  Direc- 
tor of  the  Boston  School  of  Gymnastics. 
•  (Double  Number.) 

Christianity  and  Agnosticism.  A 
Controversy.  Consisting  of  papers 
contributed  to  The  Nineteenth  Century  by 
Henry  Wace,  D.D.,  Prof. "Thomas  H.  Hux- 
ley, The  Bishop  of  Peterborough,  W.  H. 
Mallock,  Mrg.  Humphry  Ward.  (Double 
Number.) 

f  o.  115.  Darwinism :  An  Exposition  of  the  Theory 
of  Natural  Selection,  with  some  of  its 
applications.  Part  I.  By  Alfred  Russel 
Wallace,  LL.D.,  F.L.S.,  etc.  Illustrated. 
(Double  Number.) 

Jo  116  Darwinism  :  An  Exposition  of  the 
Theory  of  Natural  Selection,  with  some 
of  its  Applications.  Part  II.  Illustrated. 
(Double  Number  ) 

So  117.  Modern  Science  and  Mod.  Thought. 

By     S.      Laiug.      Illustrated.      (Double 
Number.) 
No.  118.  Modern  Science  and  Mod.  Thought. 

Part  II.    By  S.  Laing. 

No.  119.  The  Electric  Light  and  The  Storing  of 
Electrical  Energy.  (Illustrated)  Gerald 
Molloy.  D.D.,  D.Sc. 

No.  120.  The  Modern  Theory  of  Heat  and  The 

Sun  as  a  Storehouse  ef  energy.     (Illus- 
trated.)   Gerald  Molloy,  D,D.,  D.Sc. 


No.  124.  The  Quintessence  of  Socialism. 

Prof.  A.  Schaffle. 
f  Darwinism  &  Politics.    By  David  G. 
,,     10..    j    Ritchie,  M.A. 
No.  125.  -i  AminiHtrative  Nihilism. 

[   Huxley,  F.R.S. 
No.  126.  Physiognomy  &  Expression.     By  P. 
Mantegazza.    Illustrated.    Parti.    (Dou- 
ble Number. ) 
No.  127.  Physiognomy  &  Expression.    Part  II. 

(Double  Number.) 

No.  128.  The  Industrial  Revolution.    By  Arn- 
old  Toynbee,  Tutor   of   Baliol   College, 
Oxford.    With    a   short  memoir   by   B. 
Jowett.    Part  I.    (Double  Number.) 
Jo.  129.  The  Industrial  Revolution.    Part  II. 

(Double  Number.) 
"Jo.  130.  The  Origin  of  the  Aryans.      By  Dr. 
Isaac  Taylor.    Illustrated.    Parti.    (Dou- 
ble Number!) 
.  131.  The  Origin  of  the  Aryans.     Part  II. 

(Double  Number.) 

.  132.  The  Evolution  of  Sex.      By  Prof.    P. 
Geddes  and  J.  Arthur  Thomson.    Illus- 
trated.   Parti.    (Double  Number.) 
No.  133.  The  Evolution  of  Sex.    Part  II.    (Dou- 
ble Number.) 
Jo.  134.  The  Law  of  Private  Right.    By  George 

H.  Smith.    (Double  Number.) 

No.  135.  Capital.    A  Critical  Analysis  of  Capitalist 
Production.      By    Karl   Marx.      Part   I. 
(Double  Number.) 
No.  136.  Capital.    Part  II.    (Double  Number.) 
No.  137.  Capital.    Part  III.    (Double  Number.) 
No.  138.  Capital.    Part  IV.    (Double  Number.) 
Oct.  15,  J890. 


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THE  LAND  QUESTION,  CONTAINING  : 

THE  HISTORY   OF    LANDHOLDING    IN   ENGLAND.      By 

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SCIENCE  AND  CRIME,  and 

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CURRENT  DISCUSSIONS  IN  SCIENCE,  and 

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Hecker,  M.D. 

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WOKKS    BY    PROFESSOR    HUXLEY. 

MAN'S  PLACE  IN  NATURE  (with  numerous  illustrations). 
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14  DAY  USE 

RETURN  TO  DESK  FROM  WHICH  BORROWED 

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